可以代替mysql_num_rows使用的mysql查詢

Question

我想知道如何編寫一個mysql查詢來檢查其他表中有多少條記錄嗎？

table 1 : names

id name
-- ----
4  john
5  mike
6  jenny

table 2 : cars

id name_id car
-- ------- ---
1   4      bmw
2   4      wv
3   6      honda

query = "SELECT * FROM names WHERE id = '4'" <-在該查詢中，我還想檢查cars表中有多少條記錄，或者是否有？

Answer 1

應該這樣做：

SELECT names.id, name, COUNT(car) as 'cars'
FROM names LEFT JOIN cars ON names.id = cars.nameid 
WHERE names.id = '4'
GROUP BY names.id, name

Answer 2

在額外的LEFT OUTER JOIN增加每個人的LEFT OUTER JOIN

SELECT  n.*, c.cnt
FROM    names n
        LEFT OUTER JOIN (
          SELECT  name_id, cnt = COUNT(*)
          FROM    cars
          GROUP BY
                  name_id
        ) c ON c.name_id = n.id
WHERE   ID = 4

如果性能是一個問題，則可以將條件添加到左連接中，但會犧牲可維護性。

SELECT  n.*, c.cnt
FROM    names n
        LEFT OUTER JOIN (
          SELECT  name_id, cnt = COUNT(*)
          FROM    cars
          WHERE   name_id = 4 -- Condition needs to be updated in two places
          GROUP BY
                  name_id
        ) c ON c.name_id = n.id
WHERE   ID = 4

Answer 3

SELECT n.*, COUNT(c.*) FROM names n, cars c WHERE n.id = 4

您還可以命名該列：

SELECT n.*, COUNT(c.*) AS `num_cars` FROM names n, cars c WHERE n.id = 4