Explicit vs implicit SQL joins

Question

Is there any efficiency difference in an explicit vs implicit inner join  For example   SELECT   FROM table a INNER JOIN table b ON a id   b id    vs   SELECT a    b   FROM table a  table b WHERE a id   b id

User · Answer

Personally I prefer the join syntax as its makes it clearer that the tables are joined and how they are joined  Try compare larger SQL queries where you selecting from 8 different tables and you have lots of filtering in the where  By using join syntax you separate out the parts where the tables are joined  to the part where you are filtering the rows

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lomaxx   Just to clarify  I m pretty certain that both above syntax are supported by SQL Serv 2005   The syntax below is NOT supported however  select a    b     from table a  table b   where a id    b id    Specifically  the outer join      is not supported

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Performance wise  they are exactly the same  at least in SQL Server  but be aware that they are deprecating this join syntax and it s not supported by sql server2005 out of the box    I think you are thinking of the deprecated    and    operators vs   outer join    I have just now tested the two formats given  and they work properly on a SQL Server 2008 database   In my case they yielded identical execution plans  but I couldn t confidently say that this would always be true

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On some databases  notably Oracle  the order of the joins can make a huge difference to query performance  if there are more than two tables   On one application  we had literally two orders of magnitude difference in some cases  Using the inner join syntax gives you control over this - if you use the right hints syntax   You didn t specify which database you re using  but probability suggests SQL Server or MySQL where there it makes no real difference

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In my experience  using the cross-join-with-a-where-clause syntax often produces a brain damaged execution plan  especially if you are using a Microsoft SQL product   The way that SQL Server attempts to estimate table row counts  for instance  is savagely horrible   Using the inner join syntax gives you some control over how the query is executed   So from a practical point of view  given the atavistic nature of current database technology  you have to go with the inner join

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As Leigh Caldwell has stated  the query optimizer can produce different query plans based on what functionally looks like the same SQL statement  For further reading on this  have a look at the following two blog postings -  One posting from the Oracle Optimizer Team  Another posting from the  Structured Data  blog  I hope you find this interesting

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Performance wise  it should not make any difference  The explicit join syntax seems cleaner to me as it clearly defines relationships between tables in the from clause and does not clutter up the where clause

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On MySQL 5 1 51  both queries have identical execution plans   mysql gt  explain select   from table1 a inner join table2 b on a pid   b pid   ---- ------------- ------- ------ --------------- ------ --------- -------------- ------ -------    id   select type   table   type   possible keys   key    key len   ref            rows   Extra    ---- ------------- ------- ------ --------------- ------ --------- -------------- ------ -------     1   SIMPLE        b       ALL    PRIMARY         NULL   NULL      NULL            986              1   SIMPLE        a       ref    pid             pid    4         schema b pid     70            ---- ------------- ------- ------ --------------- ------ --------- -------------- ------ -------  2 rows in set  0 02 sec   mysql gt  explain select   from table1 a  table2 b where a pid   b pid   ---- ------------- ------- ------ --------------- ------ --------- -------------- ------ -------    id   select type   table   type   possible keys   key    key len   ref            rows   Extra    ---- ------------- ------- ------ --------------- ------ --------- -------------- ------ -------     1   SIMPLE        b       ALL    PRIMARY         NULL   NULL      NULL            986              1   SIMPLE        a       ref    pid             pid    4         schema b pid     70            ---- ------------- ------- ------ --------------- ------ --------- -------------- ------ -------  2 rows in set  0 00 sec    table1 has 166208 rows  table2 has about 1000 rows   This is a very simple case  it doesn t by any means prove that the query optimizer wouldn t get confused and generate different plans in a more complicated case

User · Answer

Performance wise  they are exactly the same  at least in SQL Server    PS  Be aware that the IMPLICIT OUTER JOIN syntax is deprecated since SQL Server 2005   The IMPLICIT INNER JOIN syntax as used in the question is still supported   Deprecation of  Old Style  JOIN Syntax  Only A Partial Thing

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Basically  the difference between the two is that one is written in the old way  while the other is written in the modern way  Personally  I prefer the modern script using the inner  left  outer  right definitions because they are more explanatory and makes the code more readable   When dealing with inner joins there is no real difference in readability neither  however  it may get complicated when dealing with left and right joins as in the older method you would get something like this   SELECT    FROM table a  table b WHERE a id   b id        The above is the old way how a left join is written as opposed to the following   SELECT    FROM table a  LEFT JOIN table b ON a id   b id    As you can visually see  the modern way of how the script is written makes the query more readable   By the way same goes for right joins and a little more complicated for outer joins    Going back to the boiler plate  it doesn t make a difference to the SQL compiler how the query is written as it handles them in the same way  I ve seen a mix of both in Oracle databases which have had many people writing into it  both elder and younger ones  Again  it boils down to how readable the script is and the team you are developing with

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The second syntax has the unwanted possibility of a cross join  you can add tables to the FROM part without corresponding WHERE clause  This is considered harmful

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The first answer you gave uses what is known as ANSI join syntax  the other is valid and will work in any relational database   I agree with grom that you should use ANSI join syntax  As they said  the main reason is for clarity  Rather than having a where clause with lots of predicates  some of which join tables and others restricting the rows returned with the ANSI join syntax you are making it blindingly clear which conditions are being used to join your tables and which are being used to restrict the results

[sql] Explicit vs implicit SQL joins

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