What is the difference between JOIN and UNION

Question

What is the difference between JOIN and UNION  Can I have an example

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I like to think of the general difference as being    JOINS join tables  UNION  et all  combines queries

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UNION combines the results of two or more queries into a single result set that includes all the rows that belong to all queries in the union   By using JOINs  you can retrieve data from two or more tables based on logical relationships between the tables  Joins indicate how SQL should use data from one table to select the rows in another table   The UNION operation is different from using JOINs that combine columns from two tables   UNION Example   SELECT 1 AS  Column1   2 AS  Column2  UNION SELECT 3 AS  Column1   4 AS  Column2    Output   Column1    Column2 ------------------- 1          2 3          4   JOIN Example   SELECT a Column1  b Column2 FROM TableA a INNER JOIN TableB b ON a Id   b AFKId   This will output all the rows from both the tables for which the condition a Id   b AFKId is true

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UNION puts lines from queries after each other  while JOIN makes a cartesian product and subsets it -- completely different operations  Trivial example of UNION   mysql gt  SELECT 23 AS bah     - gt  UNION     - gt  SELECT 45 AS bah   -----    bah    -----     23       45     -----  2 rows in set  0 00 sec    similary trivial example of JOIN   mysql gt  SELECT   FROM      - gt   SELECT 23 AS bah  AS foo      - gt  JOIN      - gt   SELECT 45 AS bah  AS bar     - gt  ON  33 33    ----- -----    foo   bar    ----- -----     23    45     ----- -----  1 row in set  0 01 sec

User · Answer

You  may see the same schematic explanations for both  but these are totally confusing   For UNION     For JOIN

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They re completely different things   A join allows you to relate similar data in different tables   A union returns the results of two different queries as a single recordset

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Union Operation is combined result of the Vertical Aggregate of the rows  Union Operation is combined result of the Horizontal Aggregate of the Columns

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Ussing UNION  UNION  is combines the results of two or more queries into a single result set that includes all the rows that belong to all queries in the union    UNION Example  SELECT 121 AS  Column1   221 AS  Column2  UNION SELECT 321 AS  Column1   422 AS  Column2     Output   Column1    Column2 ------------------- 121          221 321          422   Ussing JOINs  JOINs  you can retrieve data from two or more tables based on logical relationships between the tables    JOIN Example  SELECT a Column1  b Column2 FROM TblA a INNER JOIN TblB b ON a Id   b id

User · Answer

In the abstract  they are similar  in that two tables or result sets are being combined   but UNION is really for combining result sets with the SAME NUMBER OF COLUMNS with the COLUMNS HAVING SIMILAR DATA TYPES  The STRUCTURE is the same  only new rows are being added    In joins  you can combine tables result sets with any possible structure  including a cartesian join where there are NO shared similar columns

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The UNION operator is just for combining two or more SELECT statements   While JOIN is for selecting rows from each table  either by the inner  outer  left or right method   Refer to here and here   There is a better explanation with examples

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JOIN   A join is used for displaying columns with the same or different names from different tables  The output displayed will have all the columns shown individually  That is  the columns will be aligned next to each other   UNION   The UNION set operator is used for combining data from two tables which have columns with the same datatype  When a UNION is performed the data from both tables will be collected in a single column having the same datatype   For example   See the two tables shown below   Table t1 Articleno article price manufacturer id 1 hammer 3   1 2 screwdriver 5   2  Table t2 manufacturer id manufacturer 1 ABC Gmbh 2 DEF Co KG   Now for performing a JOIN type the query is shown below   SELECT articleno  article  manufacturer FROM t1 JOIN t2 ON  t1 manufacturer id   t2 manufacturer id    articelno article manufacturer 1 hammer ABC GmbH 2 screwdriver DEF Co KG   That is a join   UNION means that you have to tables or resultset with the same amount and type of columns and you add this to tables resultsets together  Look at this example   Table year2006 Articleno article price manufacturer id 1 hammer 3   1 2 screwdriver 5   2  Table year2007 Articleno article price manufacturer id 1 hammer 6   3 2 screwdriver 7   4  SELECT articleno  article  price  manufactruer id FROM year2006 UNION SELECT articleno  article  price  manufacturer id FROM year2007  articleno article price manufacturer id 1 hammer 3   1 2 screwdriver 5   2 1 hammer 6   3 2 screwdriver 7   4

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Joins and unions can be used to combine data from one or more tables  The difference lies in how the data is combined   In simple terms  joins combine data into new columns  If two tables are joined together  then the data from the first table is shown in one set of column alongside the second table   s column in the same row   Unions combine data into new rows  If two tables are    unioned    together  then the data from the first table is in one set of rows  and the data from the second table in another set  The rows are in the same result

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Remember that union will merge results  SQL nbsp Server to be sure  feature or bug    select 1 as id  3 as value union select 1 as id  3 as value   id value  1 3  select   from  select 1 as id  3 as value  t1 inner join  select 1 as id  3 as value  t2 on t1 id   t2 id   id value id value  1 3 1 3

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Joins and Unions can be used to combine data from one or more tables   The difference lies in how the data is combined   In simple terms  joins combine data into new columns   If two tables are joined together  then the data from the first table is shown in one set of column alongside the second table   s column in the same row   Unions combine data into new rows   If two tables are    unioned    together  then the data from the first table is in one set of rows  and the data from the second table in another set   The rows are in the same result   Here is a visual depiction of a join   Table A and B   s columns are combined into a single result     Each row in the result contains columns from BOTH table A and B   Rows are created when columns from one table match columns from another   This match is called the join condition   This makes joins really great for looking up values and including them in results   This is usually the result of denormalizing  reversing normalization  and involves using the foreign key in one table to look up column values by using the primary key in another   Now compare the above depiction with that of a union   In a union each row within the result is from one table OR the other   In a union  columns aren   t combined to create results  rows are combined     Both joins and unions can be used to combine data from one or more tables into a single results   They both go about this is different ways   Whereas a join is used to combine columns from different tables  the union is used to combine rows   Source

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Union makes two queries look like one  Joins are for examining two or more tables in a single query statement

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1  The SQL Joins clause is used to combine records from two or more tables in a database  A JOIN is a means for combining fields from two tables by using values common to each    2  The SQL UNION operator combines the result of two or more SELECT statements  Each SELECT statement within the UNION must have the same number of columns  The columns must also have similar data types  Also  the columns in each SELECT statement must be in the same order   for example  table 1 customers table 2 orders  inner join   SELECT ID  NAME  AMOUNT  DATE  FROM CUSTOMERS   INNER JOIN ORDERS   ON CUSTOMERS ID   ORDERS CUSTOMER ID    union   SELECT ID  NAME  AMOUNT  DATE   FROM CUSTOMERS   LEFT JOIN ORDERS   ON CUSTOMERS ID   ORDERS CUSTOMER ID   UNION   SELECT ID  NAME  AMOUNT  DATE   FROM CUSTOMERS   RIGHT JOIN ORDERS   ON CUSTOMERS ID   ORDERS CUSTOMER ID

[sql] What is the difference between JOIN and UNION?

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