Question

除了使用SELECT或UNION之外，还有其他方法可以在MySQL查询中合并超过2个UNION ALL语句吗？我已尝试使用UNION和UNION ALL但我的查询加载速度太慢。这是我的问题：

SELECT 'AVAILABLE' AS STATUS, count(id_status) as BIL
FROM 
book_records AS b, book_class AS c
WHERE p.id_book = k.id_book AND id_status IN ( 1 ) AND class_desc =  'NOVEL'
UNION
SELECT 'WAITING' AS STATUS, count(id_status) as BIL
FROM 
book_records AS b, book_class AS c
WHERE p.id_book = k.id_book AND id_status IN ( 2,3,5 ) AND class_desc =  'NOVEL'
UNION
SELECT 'DAMAGED' AS STATUS, count(id_status) as BIL
FROM 
book_records AS b, book_class AS c
WHERE p.id_book = k.id_book AND id_status NOT IN ( 1,2,3,5 ) AND class_desc =  'NOVEL'

有谁能告诉我如何解决问题？

Answer 1

假设p.id_book = k.id_book实际上意味着b.id_book = c.id_book，您可以尝试以下方法：

SELECT
  STATUS,
  COUNT(*) AS BIL
FROM (
  SELECT
    CASE
      WHEN id_status IN (1)       THEN 'AVAILABLE'
      WHEN id_status IN (2, 3, 5) THEN 'WAITING'
      ELSE                             'DAMAGED'
    END AS STATUS
  FROM
    book_records AS b
  INNER JOIN
    book_class AS c
  ON
    b.id_book = c.id_book
  WHERE
    class_desc =  'NOVEL'
) AS s
GROUP BY
  STATUS
;

可能更好，因为联接的结果集只会被扫描一次。

顺便说一句，在语法上，MySQL允许您在这种情况下避免使用子选择，并将GROUP BY STATUS直接应用于定义STATUS计算列的查询。但是，这不是标准语法，可能很少（如果有的话）其他SQL产品支持它。因此，避免它可能是一个好主意。

此外，关于可维护性和可读性，当查询中涉及多个表时，始终最好使用列来自的表的别名来限定列，即使列名称是唯一的。

除了使用UNION @ UNION ALL之外，在MySQL中合并2个以上的SELECT语句

1 个答案: