Question

In MySQL, how can I find all rows whose attribute1 is the same as a particular row’s attribute1? I thought about doing

SELECT 
    t1.id 
FROM 
    t AS t1
  , t AS t2 
WHERE 
    t2.id=123 
AND t1.a=t2.a;

but it has been running for eons.

Answer 1

This should work to return the required rows.

SELECT t1.id 
 FROM t AS t1
 JOIN t AS t2 ON (t1.a = t2.a and t1.id <> t2.id)
WHERE t2.id=123;

How many rows are in your table? Is the “a” column indexed? Adding an index should speed up the join.

Here’s an example on SQLFiddle.