将mysql子查询存储在变量中

Question

是否可以以某种方式存储mysql子查询,如果它将再次用作子查询？据推测,这将产生更清晰的代码以及节省解析开销.

例如,在以下外部联接中

SELECT * FROM t1
LEFT JOIN (SELECT * FROM t2 WHERE t2.foo=='bar') ON t1.id = t2.id
UNION
SELECT * FROM t1
RIGHT JOIN (SELECT * FROM t2 WHERE t2.foo=='bar') ON t1.id = t2.id

不重复会很好(SELECT * FROM t2 WHERE t2.foo=='bar').

Answer 1

当然这样做

SET @condition := (SELECT * FROM t2 WHERE t2.foo=='bar');
SELECT * FROM t1
LEFT JOIN (@condition) ON t1.id = t2.id
UNION
SELECT * FROM t1
RIGHT JOIN (@condition) ON t1.id = t2.id

Answer 2

不，你不能。如果MySQL具有CTE（公用表表达式），则可以使用以下命令：

WITH tmp AS
  (SELECT * FROM t2 WHERE t2.foo = 'bar')
SELECT * FROM t1
  LEFT JOIN tmp ON t1.id = tmp.id
UNION
SELECT * FROM t1
  RIGHT JOIN tmp ON t1.id = tmp.id

如果MySQL有FULL JOIN（可惜，那也没有！），您可以使用以下代码：

SELECT * FROM t1
  FULL JOIN (SELECT * FROM t2 WHERE t2.foo = 'bar') tmp
    ON t1.id = tmp.id