通过一次查询选择不同行上的多个字段

Question

With one MYSQL query only, how can I check if a buyer and a seller have both bought from each other ?

Table : purchase

purchase_id     |   seller       |   buyer
       12       |      2         |     5
       29       |      5         |     2

If the query returns a result (which would be the case here), then a specific div would appear on the page.

Answer 1

SELECT (COUNT(*) >= 2) AS Result
FROM   purchase a
       INNER JOIN purchase b
         ON a.seller = b.buyer AND
            a.buyer = b.seller
WHERE  (a.seller = 2 AND a.buyer = 5) OR
       (a.seller = 5 AND a.buyer = 2)

• SQLFiddle Demo

just replace 2 and 5 with your variables.