SQL left join vs multiple tables on FROM line

Question

Most SQL dialects accept both the following queries   SELECT a foo  b foo FROM a  b WHERE a x   b x  SELECT a foo  b foo FROM a LEFT JOIN b ON a x   b x   Now obviously when you need an outer join  the second syntax is required  But when doing an inner join why should I prefer the second syntax to the first  or vice versa

User · Answer

Basically  when your FROM clause lists tables like so   SELECT   FROM   tableA  tableB  tableC   the result is a cross product of all the rows in tables A  B  C  Then you apply the restriction WHERE tableA id   tableB a id which will throw away a huge number of rows  then further     AND tableB id   tableC b id and you should then get only those rows you are really interested in   DBMSs know how to optimise this SQL so that the performance difference to writing this using JOINs is negligible  if any   Using the JOIN notation makes the SQL statement more readable  IMHO  not using joins turns the statement into a mess   Using the cross product  you need to provide join criteria in the WHERE clause  and that s the problem with the notation  You are crowding your WHERE clause with stuff like      tableA id   tableB a id  AND tableB id   tableC b id    which is only used to restrict the cross product  WHERE clause should only contain RESTRICTIONS to the resultset  If you mix table join criteria with resultset restrictions  you  and others  will find your query harder to read  You should definitely use JOINs and keep the FROM clause a FROM clause  and the WHERE clause a WHERE clause

User · Answer

The SELECT   FROM table1  table2      syntax is ok for a couple of tables  but it becomes exponentially  not necessarily a mathematically accurate statement  harder and harder to read as the number of tables increases    The JOIN syntax is harder to write  at the beginning   but it makes it explicit what criteria affects which tables  This makes it much harder to make a mistake   Also  if all the joins are INNER  then both versions are equivalent  However  the moment you have an OUTER join anywhere in the statement  things get much more complicated and it s virtually guarantee that what you write won t be querying what you think you wrote

User · Answer

When you need an outer join the second syntax is not always required   Oracle   SELECT a foo  b foo   FROM a  b  WHERE a x   b x      MSSQLServer  although it s been deprecated in 2000 version  Sybase   SELECT a foo  b foo   FROM a  b  WHERE a x    b x   But returning to your question  I don t know the answer  but it is probably related to the fact that a join is more natural  syntactically  at least  than adding an expression to a where clause when you are doing exactly that  joining

User · Answer

The second is preferred because it is far less likely to result in an accidental cross join by forgetting to put inthe where clause  A join with no on clause will fail the syntax check  an old style join with no where clause will not fail  it will do a cross join   Additionally when you later have to a left join  it is helpful for maintenance that they all be in the same structure  And the old syntax has been out of date since 1992  it is well past time to stop using it   Plus I have found that many people who exclusively use the first syntax don t really understand joins and understanding joins is critical to getting correct results when querying

User · Answer

The first way is the older standard  The second method was introduced in SQL-92  http   en wikipedia org wiki SQL  The complete standard can be viewed at http   www contrib andrew cmu edu  shadow sql sql1992 txt    It took many years before database companies adopted the SQL-92 standard   So the reason why the second method is preferred  it is the SQL standard according the ANSI and ISO standards committee

User · Answer

To the database  they end up being the same  For you  though  you ll have to use that second syntax in some situations  For the sake of editing queries that end up having to use it  finding out you needed a left join where you had a straight join   and for consistency  I d pattern only on the 2nd method  It ll make reading queries easier

User · Answer

The JOIN syntax keeps conditions near the table they apply to  This is especially useful when you join a large amount of tables   By the way  you can do an outer join with the first syntax too   WHERE a x   b x      Or  WHERE a x    b x   Or  WHERE a x   b x or a x not in  select x from b

User · Answer

I think there are some good reasons on this page to adopt the second method -using explicit JOINs   The clincher though is that when the JOIN criteria are removed from the WHERE clause it becomes much easier to see the remaining selection criteria in the WHERE clause   In really complex SELECT statements it becomes much easier for a reader to understand what is going on

User · Answer

The old syntax  with just listing the tables  and using the WHERE clause to specify the join criteria  is being deprecated in most modern databases   It s not just for show  the old syntax has the possibility of being ambiguous when you use both INNER and OUTER joins in the same query   Let me give you an example   Let s suppose you have 3 tables in your system   Company Department Employee   Each table contain numerous rows  linked together  You got multiple companies  and each company can have multiple departments  and each department can have multiple employees   Ok  so now you want to do the following      List all the companies  and include all their departments  and all their employees  Note that some companies don t have any departments yet  but make sure you include them as well  Make sure you only retrieve departments that have employees  but always list all companies    So you do this   SELECT   -- for simplicity FROM Company  Department  Employee WHERE Company ID    Department CompanyID   AND Department ID   Employee DepartmentID   Note that the last one there is an inner join  in order to fulfill the criteria that you only want departments with people   Ok  so what happens now  Well  the problem is  it depends on the database engine  the query optimizer  indexes  and table statistics  Let me explain   If the query optimizer determines that the way to do this is to first take a company  then find the departments  and then do an inner join with employees  you re not going to get any companies that don t have departments   The reason for this is that the WHERE clause determines which rows end up in the final result  not individual parts of the rows   And in this case  due to the left join  the Department ID column will be NULL  and thus when it comes to the INNER JOIN to Employee  there s no way to fulfill that constraint for the Employee row  and so it won t appear   On the other hand  if the query optimizer decides to tackle the department-employee join first  and then do a left join with the companies  you will see them   So the old syntax is ambiguous  There s no way to specify what you want  without dealing with query hints  and some databases have no way at all   Enter the new syntax  with this you can choose   For instance  if you want all companies  as the problem description stated  this is what you would write   SELECT   FROM Company      LEFT JOIN            Department INNER JOIN Employee ON Department ID   Employee DepartmentID        ON Company ID   Department CompanyID   Here you specify that you want the department-employee join to be done as one join  and then left join the results of that with the companies   Additionally  let s say you only want departments that contains the letter X in their name  Again  with old style joins  you risk losing the company as well  if it doesn t have any departments with an X in its name  but with the new syntax  you can do this   SELECT   FROM Company      LEFT JOIN            Department INNER JOIN Employee ON Department ID   Employee DepartmentID        ON Company ID   Department CompanyID AND Department Name LIKE   X     This extra clause is used for the joining  but is not a filter for the entire row  So the row might appear with company information  but might have NULLs in all the department and employee columns for that row  because there is no department with an X in its name for that company  This is hard with the old syntax   This is why  amongst other vendors  Microsoft has deprecated the old outer join syntax  but not the old inner join syntax  since SQL Server 2005 and upwards  The only way to talk to a database running on Microsoft SQL Server 2005 or 2008  using the old style outer join syntax  is to set that database in 8 0 compatibility mode  aka SQL Server 2000    Additionally  the old way  by throwing a bunch of tables at the query optimizer  with a bunch of WHERE clauses  was akin to saying  here you are  do the best you can   With the new syntax  the query optimizer has less work to do in order to figure out what parts goes together   So there you have it   LEFT and INNER JOIN is the wave of the future

User · Answer

I hear a lot of people complain the first one is too difficult to understand and that it is unclear   I don t see a problem with it  but after having that discussion  I use the second one even on INNER JOINS for clarity

User · Answer

Well the first and second queries may yield different results because a LEFT JOIN includes all records from the first table  even if there are no corresponding records in the right table

[sql] SQL left join vs multiple tables on FROM line?

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