Question

我有两张桌子：

all_countries

╔════╦═════════════╗
║ ID ║   COUNTRY   ║
╠════╬═════════════╣
║  1 ║ USA         ║
║  2 ║ China       ║
║  3 ║ India       ║
║  4 ║ France      ║
║  5 ║ UK          ║
║  6 ║ Australia   ║
║  7 ║ Philippines ║
╚════╩═════════════╝

supported_countries

╔════════════╦═══════════╦═══════════╗
║ COUNTRY_ID ║  COUNTRY  ║ FILTER_ID ║
╠════════════╬═══════════╬═══════════╣
║          1 ║ USA       ║         1 ║
║          2 ║ China     ║         1 ║
║          3 ║ India     ║         1 ║
║          4 ║ France    ║         2 ║
║          1 ║ USA       ║         2 ║
║          6 ║ Australia ║         2 ║
╚════════════╩═══════════╩═══════════╝

查询：

SELECT a.* 
FROM all_countries a
LEFT JOIN supported_countries s
  ON a.id = s.country_id
WHERE s.country_id IS NULL;

因此，按照上面的例子我应该得到。

UK
Philippines

它工作正常，但是，如果我过滤国家示例我选择过滤= 2。我得到零结果。

SELECT a.* 
FROM all_countries a
LEFT JOIN supported_countries s
  ON a.id = s.country_id
WHERE s.filter = 2 AND s.country_id IS NULL;

如果filter = 2，

应该是我想得到这个结果。

╔════╦═════════════╗
║ ID ║   COUNTRY   ║
╠════╬═════════════╣
║  2 ║ China       ║
║  3 ║ India       ║
║  5 ║ UK          ║
║  7 ║ Philippines ║
╚════╩═════════════╝

我正在进行正确的查询吗？

Answer 1

在ON子句

上添加条件

SELECT  a.* 
FROM    all_countries a
        LEFT JOIN supported_countries s
           ON a.id = s.country_id AND 
              s.filter_ID = 2        -- <<=== HERE
WHERE   s.country_id IS NULL;

SQLFiddle Demo

结果：

╔════╦═════════════╗
║ ID ║   COUNTRY   ║
╠════╬═════════════╣
║  2 ║ China       ║
║  3 ║ India       ║
║  5 ║ UK          ║
║  7 ║ Philippines ║
╚════╩═════════════╝

获取其他表中不存在的记录

1 个答案: