左联接优化

Question

I'm wondering if it's better to do joined query like that :

SELECT * FROM table1
    LEFT JOIN table2 ON table1.field1=table2.id
    AND table2.field2="mystring"
WHERE table2.field2="mystring"

Instead of simply doing it :

SELECT * FROM table1
    LEFT JOIN table2 ON table1.field1=table2.id
WHERE table2.field2="mystring"

Is the MySQL engine doing this optimizations for me or is it better to do it explicitly ?

Does :

SELECT * FROM table1
    JOIN table2 ON table2.field2="mystring" AND table2.id=table1.field1

has the same effect, will results be identical ?

Edit: Some more precisions, my questions are :

• i assume the 3 above queries are equivalent in term of given results. Are they really (especially the third) ?

• i'm wondering if the engine treat them equally in terms of performances (in other words do i have to worry wich one to use) ?

Answer 1

This will still show records of table1 if table2.field2!="mystring"

SELECT * FROM table1
    LEFT JOIN table2 ON table1.field1=table2.id
    AND table2.field2="mystring"

This will not show records of table1 if table2.field2!="mystring"

SELECT * FROM table1
    LEFT JOIN table2 ON table1.field1=table2.id
WHERE table2.field2="mystring"

The output can be different. The first query in this situation could show more results then the second.