結合兩個SQL語句

Question

將兩個SQL語句合並為一個的最佳方法是什么？

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

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

提前致謝！

Answer 1

假設兩個結果集每個只包含一行，最簡單的方法是交叉連接：

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

這里的主要好處是將所有記錄放在一行上，而不是加入每個表的組件記錄。

在聚合之前涉及聯接的答案將引入顯着的開銷，這將對性能產生巨大影響。

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

應該做的工作。

Answer 4

你可以用可怕的JOIN語法做到這一點：

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

但是你應該使用正確的JOIN語法

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

無論是

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。 我假設'room'表有一個唯一的key字段，room_grade可以有相同的字段。 加入兩個這樣的東西吧。

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