What is the difference between Left Right Outer and Inner Joins

Question

I am wondering how to differentiate all these different joins

User · Answer

LEFT JOIN and RIGHT JOIN are types of OUTER JOINs    INNER JOIN is the default -- rows from both tables must match the join condition

User · Answer

There are only 4 kinds    Inner join  The most common type   An output row is produced for every pair of input rows that match on the join conditions  Left outer join  The same as an inner join  except that if there is any row for which no matching row in the table on the right can be found  a row is output containing the values from the table on the left  with NULL for each value in the table on the right   This means that every row from the table on the left will appear at least once in the output    Right outer join  The same as a left outer join  except with the roles of the tables reversed  Full outer join  A combination of left and right outer joins   Every row from both tables will appear in the output at least once    A  cross join  or  cartesian join  is simply an inner join for which no join conditions have been specified  resulting in all pairs of rows being output   Thanks to RusselH for pointing out FULL joins  which I d omitted

User · Answer

Simple Example  Let s say you have a Students table  and a Lockers table  In SQL  the first table you specify in a join  Students  is the LEFT table  and the second one  Lockers  is the RIGHT table  Each student can be assigned to a locker  so there is a LockerNumber column in the Student table  More than one student could potentially be in a single locker  but especially at the beginning of the school year  you may have some incoming students without lockers and some lockers that have no students assigned  For the sake of this example  let s say you have 100 students  70 of which have lockers  You have a total of 50 lockers  40 of which have at least 1 student and 10 lockers have no student  INNER JOIN is equivalent to  quot show me all students with lockers quot   Any students without lockers  or any lockers without students are missing  Returns 70 rows LEFT OUTER JOIN would be  quot show me all students  with their corresponding locker if they have one quot    This might be a general student list  or could be used to identify students with no locker  Returns 100 rows RIGHT OUTER JOIN would be  quot show me all lockers  and the students assigned to them if there are any quot    This could be used to identify lockers that have no students assigned  or lockers that have too many students  Returns 80 rows  list of 70 students in the 40 lockers  plus the 10 lockers with no student  FULL OUTER JOIN would be silly and probably not much use   Something like  quot show me all students and all lockers  and match them up where you can quot  Returns 110 rows  all 100 students  including those without lockers  Plus the 10 lockers with no student  CROSS JOIN is also fairly silly in this scenario  It doesn t use the linked lockernumber field in the students table  so you basically end up with a big giant list of every possible student-to-locker pairing  whether or not it actually exists  Returns 5000 rows  100 students x 50 lockers   Could be useful  with filtering  as a starting point to match up the new students with the empty lockers

User · Answer

Check out Join  SQL  on Wikipedia   Inner join - Given two tables an inner join returns all rows that exist in both tables left   right  outer  join - Given two tables returns all rows that exist in either the left or right table of your join  plus the rows from the other side will be returned when the join clause is a match or null will be returned for those columns Full Outer - Given two tables returns all rows  and will return nulls when either the left or right column is not there Cross Joins - Cartesian join and can be dangerous if not used carefully

User · Answer

Simple Example  Let s say you have a Students table  and a Lockers table  In SQL  the first table you specify in a join  Students  is the LEFT table  and the second one  Lockers  is the RIGHT table  Each student can be assigned to a locker  so there is a LockerNumber column in the Student table  More than one student could potentially be in a single locker  but especially at the beginning of the school year  you may have some incoming students without lockers and some lockers that have no students assigned  For the sake of this example  let s say you have 100 students  70 of which have lockers  You have a total of 50 lockers  40 of which have at least 1 student and 10 lockers have no student  INNER JOIN is equivalent to  quot show me all students with lockers quot   Any students without lockers  or any lockers without students are missing  Returns 70 rows LEFT OUTER JOIN would be  quot show me all students  with their corresponding locker if they have one quot    This might be a general student list  or could be used to identify students with no locker  Returns 100 rows RIGHT OUTER JOIN would be  quot show me all lockers  and the students assigned to them if there are any quot    This could be used to identify lockers that have no students assigned  or lockers that have too many students  Returns 80 rows  list of 70 students in the 40 lockers  plus the 10 lockers with no student  FULL OUTER JOIN would be silly and probably not much use   Something like  quot show me all students and all lockers  and match them up where you can quot  Returns 110 rows  all 100 students  including those without lockers  Plus the 10 lockers with no student  CROSS JOIN is also fairly silly in this scenario  It doesn t use the linked lockernumber field in the students table  so you basically end up with a big giant list of every possible student-to-locker pairing  whether or not it actually exists  Returns 5000 rows  100 students x 50 lockers   Could be useful  with filtering  as a starting point to match up the new students with the empty lockers

User · Answer

There are three basic types of join    INNER join compares two tables and only returns results where a match exists   Records from the 1st table are duplicated when they match multiple results in the 2nd   INNER joins tend to make result sets smaller  but because records can be duplicated this isn t guaranteed  CROSS join compares two tables and return every possible combination of rows from both tables   You can get a lot of results from this kind of join that might not even be meaningful  so use with caution  OUTER join compares two tables and returns data when a match is available or NULL values otherwise   Like with INNER join  this will duplicate rows in the one table when it matches multiple records in the other table   OUTER joins tend to make result sets larger  because they won t by themselves remove any records from the set   You must also qualify an OUTER join to determine when and where to add the NULL values    LEFT means keep all records from the 1st table no matter what and insert NULL values when the 2nd table doesn t match    RIGHT means the opposite  keep all records from the 2nd table no matter what and insert NULL values whent he 1st table doesn t match    FULL means keep all records from both tables  and insert a NULL value in either table if there is no match     Often you see will the OUTER keyword omitted from the syntax   Instead it will just be  LEFT JOIN    RIGHT JOIN   or  FULL JOIN    This is done because INNER and CROSS joins have no meaning with respect to LEFT  RIGHT  or FULL  and so these are sufficient by themselves to unambiguously indicate an OUTER join   Here is an example of when you might want to use each type    INNER  You want to return all records from the  Invoice  table  along with their corresponding  InvoiceLines    This assumes that every valid Invoice will have at least one line  OUTER  You want to return all  InvoiceLines  records for a particular Invoice  along with their corresponding  InventoryItem  records   This is a business that also sells service  such that not all InvoiceLines will have an IventoryItem  CROSS   You have a digits table with 10 rows  each holding values  0  through  9    You want to create a date range table to join against  so that you end up with one record for each day within the range   By CROSS-joining this table with itself repeatedly you can create as many consecutive integers as you need  given you start at 10 to 1st power  each join adds 1 to the exponent    Then use the DATEADD   function to add those values to your base date for the range

User · Answer

Inner join  Only show rows  when has it data from both of the tables   Outer join   left right   Show the all result from the left   right table with the paired row s   if it exists or not

User · Answer

Simple Example  Let s say you have a Students table  and a Lockers table  In SQL  the first table you specify in a join  Students  is the LEFT table  and the second one  Lockers  is the RIGHT table  Each student can be assigned to a locker  so there is a LockerNumber column in the Student table  More than one student could potentially be in a single locker  but especially at the beginning of the school year  you may have some incoming students without lockers and some lockers that have no students assigned  For the sake of this example  let s say you have 100 students  70 of which have lockers  You have a total of 50 lockers  40 of which have at least 1 student and 10 lockers have no student  INNER JOIN is equivalent to  quot show me all students with lockers quot   Any students without lockers  or any lockers without students are missing  Returns 70 rows LEFT OUTER JOIN would be  quot show me all students  with their corresponding locker if they have one quot    This might be a general student list  or could be used to identify students with no locker  Returns 100 rows RIGHT OUTER JOIN would be  quot show me all lockers  and the students assigned to them if there are any quot    This could be used to identify lockers that have no students assigned  or lockers that have too many students  Returns 80 rows  list of 70 students in the 40 lockers  plus the 10 lockers with no student  FULL OUTER JOIN would be silly and probably not much use   Something like  quot show me all students and all lockers  and match them up where you can quot  Returns 110 rows  all 100 students  including those without lockers  Plus the 10 lockers with no student  CROSS JOIN is also fairly silly in this scenario  It doesn t use the linked lockernumber field in the students table  so you basically end up with a big giant list of every possible student-to-locker pairing  whether or not it actually exists  Returns 5000 rows  100 students x 50 lockers   Could be useful  with filtering  as a starting point to match up the new students with the empty lockers

User · Answer

SQL JOINS difference   Very simple to remember    INNER JOIN only show records common to both tables    OUTER JOIN all the content of the both tables are merged together either they are matched or not   LEFT JOIN is same as LEFT OUTER JOIN -  Select records from the first  left-most  table with matching right table records    RIGHT JOIN is same as RIGHT OUTER JOIN -  Select records from the second  right-most  table with matching left table records

User · Answer

LEFT JOIN and RIGHT JOIN are types of OUTER JOINs    INNER JOIN is the default -- rows from both tables must match the join condition

User · Answer

Check out Join  SQL  on Wikipedia   Inner join - Given two tables an inner join returns all rows that exist in both tables left   right  outer  join - Given two tables returns all rows that exist in either the left or right table of your join  plus the rows from the other side will be returned when the join clause is a match or null will be returned for those columns Full Outer - Given two tables returns all rows  and will return nulls when either the left or right column is not there Cross Joins - Cartesian join and can be dangerous if not used carefully

User · Answer

There are three basic types of join    INNER join compares two tables and only returns results where a match exists   Records from the 1st table are duplicated when they match multiple results in the 2nd   INNER joins tend to make result sets smaller  but because records can be duplicated this isn t guaranteed  CROSS join compares two tables and return every possible combination of rows from both tables   You can get a lot of results from this kind of join that might not even be meaningful  so use with caution  OUTER join compares two tables and returns data when a match is available or NULL values otherwise   Like with INNER join  this will duplicate rows in the one table when it matches multiple records in the other table   OUTER joins tend to make result sets larger  because they won t by themselves remove any records from the set   You must also qualify an OUTER join to determine when and where to add the NULL values    LEFT means keep all records from the 1st table no matter what and insert NULL values when the 2nd table doesn t match    RIGHT means the opposite  keep all records from the 2nd table no matter what and insert NULL values whent he 1st table doesn t match    FULL means keep all records from both tables  and insert a NULL value in either table if there is no match     Often you see will the OUTER keyword omitted from the syntax   Instead it will just be  LEFT JOIN    RIGHT JOIN   or  FULL JOIN    This is done because INNER and CROSS joins have no meaning with respect to LEFT  RIGHT  or FULL  and so these are sufficient by themselves to unambiguously indicate an OUTER join   Here is an example of when you might want to use each type    INNER  You want to return all records from the  Invoice  table  along with their corresponding  InvoiceLines    This assumes that every valid Invoice will have at least one line  OUTER  You want to return all  InvoiceLines  records for a particular Invoice  along with their corresponding  InventoryItem  records   This is a business that also sells service  such that not all InvoiceLines will have an IventoryItem  CROSS   You have a digits table with 10 rows  each holding values  0  through  9    You want to create a date range table to join against  so that you end up with one record for each day within the range   By CROSS-joining this table with itself repeatedly you can create as many consecutive integers as you need  given you start at 10 to 1st power  each join adds 1 to the exponent    Then use the DATEADD   function to add those values to your base date for the range

User · Answer

Making it more visible might help  One example   Table 1   ID STUDENT      STUDENT NAME      1               Raony 2               Diogo 3               Eduardo 4               Luiz   Table 2   ID STUDENT        LOCKER  3               l1 4               l2 5               l3   What I get when I do   -Inner join of Table 1 and Table 2        - Inner join returns both tables merged only when the key         ID STUDENT  exists in both tables      ID STUDENT       STUDENT NAME      LOCKER             3               Eduardo          l1         4               Luiz             l2  -Left join of Table 1 and Table 2       - Left join merges both tables with all records form table 1  in        other words  there might be non-populated fields from table 2      ID ESTUDANTE    NOME ESTUDANTE     LOCKER             1               Raony            -         2               Diogo            -         3               Eduardo          l1         4               Luiz             l2  -Right join of table 1 and table 2       - Right join merges both tables with all records from table 2  in        other words  there might be non-populated fields from table 1      ID STUDENT        STUDENT NAME     LOCKER             3               Eduardo          l1         4               Luiz             l2         5               -                l3  -Outter join of table 1 and table 2       - Returns all records from both tables  in other words  there       might be non-populated fields either from table 1 or 2       ID STUDENT        STUDENT NAME     LOCKER            1               Raony            -         2               Diogo            -         3               Eduardo          l1         4               Luiz             l2         5               -                l3

User · Answer

Check out Join  SQL  on Wikipedia   Inner join - Given two tables an inner join returns all rows that exist in both tables left   right  outer  join - Given two tables returns all rows that exist in either the left or right table of your join  plus the rows from the other side will be returned when the join clause is a match or null will be returned for those columns Full Outer - Given two tables returns all rows  and will return nulls when either the left or right column is not there Cross Joins - Cartesian join and can be dangerous if not used carefully

User · Answer

Inner join  Only show rows  when has it data from both of the tables   Outer join   left right   Show the all result from the left   right table with the paired row s   if it exists or not

User · Answer

There are only 4 kinds    Inner join  The most common type   An output row is produced for every pair of input rows that match on the join conditions  Left outer join  The same as an inner join  except that if there is any row for which no matching row in the table on the right can be found  a row is output containing the values from the table on the left  with NULL for each value in the table on the right   This means that every row from the table on the left will appear at least once in the output    Right outer join  The same as a left outer join  except with the roles of the tables reversed  Full outer join  A combination of left and right outer joins   Every row from both tables will appear in the output at least once    A  cross join  or  cartesian join  is simply an inner join for which no join conditions have been specified  resulting in all pairs of rows being output   Thanks to RusselH for pointing out FULL joins  which I d omitted

User · Answer

At first you have to understand what does join do  We connect multiple table and get specific result from the joined tables  The simplest way to do this is cross join    Lets say tableA has two column A and B  And tableB has three column C and D  If we apply cross join it will produce lot of meaningless row  Then we have to match using primary key to get actual data   Left  it will return all records from left table and matched record from right table   Right  it will return opposite to Left join  It will return all records from right table and matched records from left table   Inner  This is like intersection  It will return only matched records from both table   Outer  And this is like union  It will return all available record from both table   Some times we don t need all of the data  and also we should need only common data or records  we can easily get it using these join methods  Remember left and right join also are outer join    You can get all records just using cross join  But it could be expensive when it comes to millions of records  So make it simple by using left  right  inner or outer join   thanks

User · Answer

There are three basic types of join    INNER join compares two tables and only returns results where a match exists   Records from the 1st table are duplicated when they match multiple results in the 2nd   INNER joins tend to make result sets smaller  but because records can be duplicated this isn t guaranteed  CROSS join compares two tables and return every possible combination of rows from both tables   You can get a lot of results from this kind of join that might not even be meaningful  so use with caution  OUTER join compares two tables and returns data when a match is available or NULL values otherwise   Like with INNER join  this will duplicate rows in the one table when it matches multiple records in the other table   OUTER joins tend to make result sets larger  because they won t by themselves remove any records from the set   You must also qualify an OUTER join to determine when and where to add the NULL values    LEFT means keep all records from the 1st table no matter what and insert NULL values when the 2nd table doesn t match    RIGHT means the opposite  keep all records from the 2nd table no matter what and insert NULL values whent he 1st table doesn t match    FULL means keep all records from both tables  and insert a NULL value in either table if there is no match     Often you see will the OUTER keyword omitted from the syntax   Instead it will just be  LEFT JOIN    RIGHT JOIN   or  FULL JOIN    This is done because INNER and CROSS joins have no meaning with respect to LEFT  RIGHT  or FULL  and so these are sufficient by themselves to unambiguously indicate an OUTER join   Here is an example of when you might want to use each type    INNER  You want to return all records from the  Invoice  table  along with their corresponding  InvoiceLines    This assumes that every valid Invoice will have at least one line  OUTER  You want to return all  InvoiceLines  records for a particular Invoice  along with their corresponding  InventoryItem  records   This is a business that also sells service  such that not all InvoiceLines will have an IventoryItem  CROSS   You have a digits table with 10 rows  each holding values  0  through  9    You want to create a date range table to join against  so that you end up with one record for each day within the range   By CROSS-joining this table with itself repeatedly you can create as many consecutive integers as you need  given you start at 10 to 1st power  each join adds 1 to the exponent    Then use the DATEADD   function to add those values to your base date for the range

User · Answer

LEFT JOIN and RIGHT JOIN are types of OUTER JOINs    INNER JOIN is the default -- rows from both tables must match the join condition

User · Answer

Check out Join  SQL  on Wikipedia   Inner join - Given two tables an inner join returns all rows that exist in both tables left   right  outer  join - Given two tables returns all rows that exist in either the left or right table of your join  plus the rows from the other side will be returned when the join clause is a match or null will be returned for those columns Full Outer - Given two tables returns all rows  and will return nulls when either the left or right column is not there Cross Joins - Cartesian join and can be dangerous if not used carefully

User · Answer

There are only 4 kinds    Inner join  The most common type   An output row is produced for every pair of input rows that match on the join conditions  Left outer join  The same as an inner join  except that if there is any row for which no matching row in the table on the right can be found  a row is output containing the values from the table on the left  with NULL for each value in the table on the right   This means that every row from the table on the left will appear at least once in the output    Right outer join  The same as a left outer join  except with the roles of the tables reversed  Full outer join  A combination of left and right outer joins   Every row from both tables will appear in the output at least once    A  cross join  or  cartesian join  is simply an inner join for which no join conditions have been specified  resulting in all pairs of rows being output   Thanks to RusselH for pointing out FULL joins  which I d omitted

User · Answer

There are only 4 kinds    Inner join  The most common type   An output row is produced for every pair of input rows that match on the join conditions  Left outer join  The same as an inner join  except that if there is any row for which no matching row in the table on the right can be found  a row is output containing the values from the table on the left  with NULL for each value in the table on the right   This means that every row from the table on the left will appear at least once in the output    Right outer join  The same as a left outer join  except with the roles of the tables reversed  Full outer join  A combination of left and right outer joins   Every row from both tables will appear in the output at least once    A  cross join  or  cartesian join  is simply an inner join for which no join conditions have been specified  resulting in all pairs of rows being output   Thanks to RusselH for pointing out FULL joins  which I d omitted

User · Answer

LEFT JOIN and RIGHT JOIN are types of OUTER JOINs    INNER JOIN is the default -- rows from both tables must match the join condition

User · Answer

Simple Example  Let s say you have a Students table  and a Lockers table  In SQL  the first table you specify in a join  Students  is the LEFT table  and the second one  Lockers  is the RIGHT table  Each student can be assigned to a locker  so there is a LockerNumber column in the Student table  More than one student could potentially be in a single locker  but especially at the beginning of the school year  you may have some incoming students without lockers and some lockers that have no students assigned  For the sake of this example  let s say you have 100 students  70 of which have lockers  You have a total of 50 lockers  40 of which have at least 1 student and 10 lockers have no student  INNER JOIN is equivalent to  quot show me all students with lockers quot   Any students without lockers  or any lockers without students are missing  Returns 70 rows LEFT OUTER JOIN would be  quot show me all students  with their corresponding locker if they have one quot    This might be a general student list  or could be used to identify students with no locker  Returns 100 rows RIGHT OUTER JOIN would be  quot show me all lockers  and the students assigned to them if there are any quot    This could be used to identify lockers that have no students assigned  or lockers that have too many students  Returns 80 rows  list of 70 students in the 40 lockers  plus the 10 lockers with no student  FULL OUTER JOIN would be silly and probably not much use   Something like  quot show me all students and all lockers  and match them up where you can quot  Returns 110 rows  all 100 students  including those without lockers  Plus the 10 lockers with no student  CROSS JOIN is also fairly silly in this scenario  It doesn t use the linked lockernumber field in the students table  so you basically end up with a big giant list of every possible student-to-locker pairing  whether or not it actually exists  Returns 5000 rows  100 students x 50 lockers   Could be useful  with filtering  as a starting point to match up the new students with the empty lockers

User · Answer

Making it more visible might help  One example   Table 1   ID STUDENT      STUDENT NAME      1               Raony 2               Diogo 3               Eduardo 4               Luiz   Table 2   ID STUDENT        LOCKER  3               l1 4               l2 5               l3   What I get when I do   -Inner join of Table 1 and Table 2        - Inner join returns both tables merged only when the key         ID STUDENT  exists in both tables      ID STUDENT       STUDENT NAME      LOCKER             3               Eduardo          l1         4               Luiz             l2  -Left join of Table 1 and Table 2       - Left join merges both tables with all records form table 1  in        other words  there might be non-populated fields from table 2      ID ESTUDANTE    NOME ESTUDANTE     LOCKER             1               Raony            -         2               Diogo            -         3               Eduardo          l1         4               Luiz             l2  -Right join of table 1 and table 2       - Right join merges both tables with all records from table 2  in        other words  there might be non-populated fields from table 1      ID STUDENT        STUDENT NAME     LOCKER             3               Eduardo          l1         4               Luiz             l2         5               -                l3  -Outter join of table 1 and table 2       - Returns all records from both tables  in other words  there       might be non-populated fields either from table 1 or 2       ID STUDENT        STUDENT NAME     LOCKER            1               Raony            -         2               Diogo            -         3               Eduardo          l1         4               Luiz             l2         5               -                l3

User · Answer

SQL JOINS difference   Very simple to remember    INNER JOIN only show records common to both tables    OUTER JOIN all the content of the both tables are merged together either they are matched or not   LEFT JOIN is same as LEFT OUTER JOIN -  Select records from the first  left-most  table with matching right table records    RIGHT JOIN is same as RIGHT OUTER JOIN -  Select records from the second  right-most  table with matching left table records

User · Answer

There are three basic types of join    INNER join compares two tables and only returns results where a match exists   Records from the 1st table are duplicated when they match multiple results in the 2nd   INNER joins tend to make result sets smaller  but because records can be duplicated this isn t guaranteed  CROSS join compares two tables and return every possible combination of rows from both tables   You can get a lot of results from this kind of join that might not even be meaningful  so use with caution  OUTER join compares two tables and returns data when a match is available or NULL values otherwise   Like with INNER join  this will duplicate rows in the one table when it matches multiple records in the other table   OUTER joins tend to make result sets larger  because they won t by themselves remove any records from the set   You must also qualify an OUTER join to determine when and where to add the NULL values    LEFT means keep all records from the 1st table no matter what and insert NULL values when the 2nd table doesn t match    RIGHT means the opposite  keep all records from the 2nd table no matter what and insert NULL values whent he 1st table doesn t match    FULL means keep all records from both tables  and insert a NULL value in either table if there is no match     Often you see will the OUTER keyword omitted from the syntax   Instead it will just be  LEFT JOIN    RIGHT JOIN   or  FULL JOIN    This is done because INNER and CROSS joins have no meaning with respect to LEFT  RIGHT  or FULL  and so these are sufficient by themselves to unambiguously indicate an OUTER join   Here is an example of when you might want to use each type    INNER  You want to return all records from the  Invoice  table  along with their corresponding  InvoiceLines    This assumes that every valid Invoice will have at least one line  OUTER  You want to return all  InvoiceLines  records for a particular Invoice  along with their corresponding  InventoryItem  records   This is a business that also sells service  such that not all InvoiceLines will have an IventoryItem  CROSS   You have a digits table with 10 rows  each holding values  0  through  9    You want to create a date range table to join against  so that you end up with one record for each day within the range   By CROSS-joining this table with itself repeatedly you can create as many consecutive integers as you need  given you start at 10 to 1st power  each join adds 1 to the exponent    Then use the DATEADD   function to add those values to your base date for the range

User · Answer

At first you have to understand what does join do  We connect multiple table and get specific result from the joined tables  The simplest way to do this is cross join    Lets say tableA has two column A and B  And tableB has three column C and D  If we apply cross join it will produce lot of meaningless row  Then we have to match using primary key to get actual data   Left  it will return all records from left table and matched record from right table   Right  it will return opposite to Left join  It will return all records from right table and matched records from left table   Inner  This is like intersection  It will return only matched records from both table   Outer  And this is like union  It will return all available record from both table   Some times we don t need all of the data  and also we should need only common data or records  we can easily get it using these join methods  Remember left and right join also are outer join    You can get all records just using cross join  But it could be expensive when it comes to millions of records  So make it simple by using left  right  inner or outer join   thanks

[sql] What is the difference between Left, Right, Outer and Inner Joins?

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