在多重连接中放置条件的位置？

Question

Say if I have this query

SELECT TableA.Id, TableA.Number, TableA.Name, TableA.HOl, TableB.Contact, TableC.activity
FROM TableA
left JOIN TableB on TableA.Id = TableB.TableA_Id 
left join TableC on TableB.userid = TableC.userid
where TableA.hol = 50 
order By TableA.Id

Is it better to but the TableA.Hol in the where or in the ON clause?

I am not sure if it makes a difference, I am trying to determine why it slow. Maybe it something else with my joins?

Answer 1

This is your query:

select TableA.Id, TableA.Number, TableA.Name, TableA.HOl, TableB.Contact, TableC.activity
from TableA left join
     TableB
     on TableA.Id = TableB.TableA_Id left join
     TableC
on TableB.userid = TableC.userid
where TableA.hol = 50 
order By TableA.Id;

A left join keeps all rows in the first table, regardless of what the on clause evaluates to. This means that a condition on the first table is effectively ignored in the on clause. Well, not exactly ignored -- the condition is false so the columns from the second table will be NULL for those rows.

So, filters on the first table in a left join should be in the where clause.

Conditions on subsequent tables should be in the on clause. Otherwise, those conditions will turn the outer join into an inner join.

Answer 2

SELECT A.Id, A.Number, A.Name, A.HOl, B.Contact, C.activity
FROM TableA A
LEFT OUTER JOIN TableB B
ON (A.Id = B.TableA_Id)
LEFT OUTER JOIN TableC C
ON (B.userid = C.userid)
AND A.hol = 50
ORDER BY A.Id

If you are referencing more than one table you can use an alias which improves readability. But this has nothing to do with performance.

Answer 3

Regardless of whether you are using JOIN or LEFT JOIN , use ON to specify how the tables are related and use WHERE to filter.

In the case of JOIN , the it does not matter where you put the filtering; it is for readability that you should follow the above rule.

In the case of LEFT JOIN , the results are likely to be different.

If you do

EXPLAIN EXTENDED SELECT ...
SHOW WARNINGS;

you can see what the SQL parser decided to do. In general, it moves ON clauses are to WHERE , indicating that it does not matter (to the semantics) which place they are. But, for LEFT JOIN , some things must remain in the ON .

Note another thing:

FROM a ...
LEFT JOIN b ...
WHERE b.foo = 123

effectively throws out the LEFT . The difference between LEFT and non- LEFT is whether you get rows of b filled with NULLs . But WHERE b.foo = 123 says you definitely do not want such rows. So, for clarity for the reader, do not say LEFT .

So, I agree with your original formulation. But I also like short aliases for all tables. Be sure to qualify all columns -- the reader may not know which table a column is in.

Your title says "multiple" joins. I discussed a single JOIN ; the lecture applies to any number of JOINs .