在 MySQL 中加入多个不同的表

Question

Example to join 4 or more tables

t1 - f1,f2,f3
t2 - f1,f4,f5
t3 - f4,f6,f7,f8
t4 - f8,f9

t1,t2,t3,t4 are 4 tables and f... are corresponding fields. Please suggest a join query which is optimized for computation and memory. Eg; I want to know how many f2 X f4 X f7 X f8.

Does the order of joining tables and size of tables have any impact on sql syntax?

I searched through multiple forums and somewhere inner join is being used and UNION and somewhere sub queries.

Please explain with syntax of MySQL.

Answer 1

Just write all the joins one after the other.

SELECT COUNT(*)
FROM t1
JOIN t2 ON t1.f1 = t2.f1
JOIN t3 ON t2.f4 = t3.f4
JOIN t4 ON t3.f8 = t4.f8

The order of the joins shouldn't matter. Make sure you have indexes on all the columns being used in the ON conditions.