Question

I use MySQL 5.1. I have two tables T1(hash, value) and T2(hash, value) and I connect them using hash. They contain:

----
T1
----
1 A
2 B
3 C
4 D

----
T2
----
1 E
1 F
3 G
4 H

My purpose is to get all rows from T1 which has connection with any row from T2.

I tried to do so with:

SELECT T1.* FROM T1 LEFT JOIN T2 ON T1.hash = T2.hash;

But I failed with two ‘A’ from T1 in the an output. Where am I wrong?

Answer 1

SELECT T1.hash, T1.value 
FROM T1 
WHERE EXISTS(
             SELECT * 
             FROM T2 
             WHERE T1.hash = T2.hash);

If you wanted to use a JOIN it would need to be

SELECT DISTINCT T1.hash, T1.value
FROM T1 
INNER JOIN T2 ON T1.hash = T2.hash;

In SQL Server the first one is more efficient. I don’t know for MySQL though.