Question

我有2个MySQL表（国家/地区）和（预订）

我想列出countries的每一行，只是说明reservations是否匹配。

在每个实例中：

reservations将有1个匹配的行，或
reservations将有许多匹配的行，或
reservations将没有匹配的行

所以我想从加入中得到的就是知道是否与reservations匹配。没别了。

这是我的查询声明：

SELECT country.countryID, reservation.citizen
FROM countries AS country
LEFT JOIN (
  SELECT reservationID
  FROM reservations
  LIMIT 1
) AS reservation ON reservation.citizen = country.countryID
ORDER BY reservation.citizen

未知栏＆＃39; reservation.citizen＆＃39;在＆＃39; on＆＃39;

结论： JOIN无法解决问题。

Answer 1

问题在于子选择。请注意，您尚未将字段公民列入预订。

SELECT country.countryID, reservation.citizen
FROM countries AS country
LEFT JOIN (
  SELECT reservationID, citizen
  FROM reservations
  LIMIT 1
) AS reservation ON reservation.citizen = country.countryID
ORDER BY reservation.citizen

另一方面，你为什么要做一个subselect？为什么限制？只查询以下内容会不会更好？

SELECT 
    country.countryID, 
    count(reservation.reservationID)
FROM 
    countries AS country
    LEFT JOIN reservations AS reservation 
    ON country.countryID = reservation.citizen
GROUP BY
    country.countryID
HAVING 
    count(reservation.reservationID) > 0
ORDER BY 
    reservation.citizen
;

Answer 2

您遇到加入问题，应该是

LEFT JOIN reservations AS reservation ON reservation.citizen = country.countryID
ORDER BY reservation.citizen

OR

LEFT JOIN (
  SELECT *
  FROM reservations
  LIMIT 1
) AS reservation ON reservation.citizen = country.countryID
 ORDER BY reservation.citizen

或者更好的LEFT OUTER JOIN以获得不匹配连接条件的行。

 LEFT OUTER JOIN reservations AS reservation ON reservation.citizen = country.countryID
ORDER BY reservation.citizen

如何加入2个MySQL表以确定连接表是否匹配

2 个答案: