结合两个SQL语句

Question

将两个SQL语句合并为一个的最佳方法是什么？

  SELECT MIN(capacity) AS mincapacity, MAX(capacity) AS maxcapacity 
  FROM room

  SELECT MIN(grade) AS mingrade, MAX(grade) AS maxgrade 
  FROM room_grade

提前致谢！

Answer 1

假设两个结果集每个只包含一行，最简单的方法是交叉连接：

select * from
(SELECT MIN(capacity) AS mincapacity, MAX(capacity) AS maxcapacity FROM room) r
cross join
(SELECT MIN(grade) AS mingrade, MAX(grade) AS maxgrade FROM room_grade) g

Answer 2

SELECT
  *
FROM
  (SELECT MIN(capacity) AS mincapacity, MAX(capacity) AS maxcapacity FROM room) AS room
CROSS JOIN
  (SELECT MIN(grade) AS mingrade, MAX(grade) AS maxgrade FROM room_grade) AS room_grade

这里的主要好处是将所有记录放在一行上，而不是加入每个表的组件记录。

在聚合之前涉及联接的答案将引入显着的开销，这将对性能产生巨大影响。

Answer 3

SELECT 'CAPACITY', MIN(capacity) AS min, MAX(capacity) AS max 
FROM room
UNION
SELECT 'GRADE', MIN(grade) AS min, MAX(grade) AS max 
FROM room_grade

应该做的工作。

Answer 4

你可以用可怕的JOIN语法做到这一点：

SELECT MIN(capacity) AS mincapacity, MAX(capacity) AS maxcapacity, MIN(grade) AS mingrade, MAX(grade) AS maxgrade
  FROM room, room_grade;

但是你应该使用正确的JOIN语法

SELECT MIN(r.capacity) AS mincapacity, MAX(r.capacity) AS maxcapacity
    , MIN(rg.grade) AS mingrade, MAX(rg.grade) AS maxgrade
  FROM room r
  JOIN room_grade rg
    ON r.key = rg.key; (whatever the key is that joins the two tables)

Answer 5

无论是

SELECT MIN(capacity) AS mincapacity, MAX(capacity) AS maxcapacity    FROM room
UNION ALL
SELECT MIN(grade) AS mingrade, MAX(grade) AS maxgrade    FROM room_grade

要么

SELECT MIN(r.capacity) AS mincapacity, MAX(r.capacity) AS maxcapacity, 
       MIN(rg.grade) AS mingrade, MAX(rg.grade) AS maxgrade    
FROM room r
INNER JOIN room_grade rg ON r.Id = rg.room_id

Answer 6

您可能想要加入ID字段。 我假设'room'表有一个唯一的key字段，room_grade可以有相同的字段。 加入两个这样的东西吧。

SELECT MIN(r.capacity) AS mincapacity, MAX(r.capacity) AS maxcapacity, MIN(g.grade) AS mingrade, MAX(g.grade) AS maxgrade
FROM room r
inner join room_grade g
on room r.roomId = g.roomId