MYSQL AND或多对多表

Question

Kind of a convoluted question but, basically I have a many to many table word_relationships :

+-----+--------+--------+
|  ID | WORD_A | WORD_B |
+-----+--------+--------+
|  1  |  784   |  893   |
+-----+--------+--------+
|  2  |  930   |  839   |
+-----+--------+--------+
|  3  |  093   |  647   |
+-----+--------+--------+

it is a list of word a and word b relationships. I need to query to find if the relationship between word_a and word_b exists. What would be the proper syntax for this if the words can be either word A OR word b?

In my head it's

w1 = 784
w2 = 893
"SELECT ID FROM word_relationships WHERE WORD_A = w1 OR WORD_B = w1 AND WORD_A = w2 OR WORD_B = w2";

This doesn't seem to work because I get back a result for any match. Anyone familiar with this situation and know the proper syntax?

Thanks in advance.

Answer 1

I think you can just put parenthesis in your where clauses to combine the ORs properly.

SELECT ID 
FROM word_relationships 
WHERE (WORD_A = w1 OR WORD_B = w1) 
AND (WORD_A = w2 OR WORD_B = w2)

I'm not totally sure I'm understanding your intention for the query though. Hope that helps.

BTW, the reason you were getting back all of the results is because of the order of precedence for SQL operators.

Since AND is evaluated before OR, it meant that by default the clause was grouped as follows:

WHERE WORD_A = w1 
OR (WORD_B = w1 AND WORD_A = w2) 
OR WORD_B = w2

which would return many more than you were expecting.

Answer 2

This should work:

SELECT 
  id 
FROM 
  word_relationships
WHERE 
 (Word_A = w1 AND Word_B = w2)
OR 
 (Word_A = w2 AND Word_B = w1)