Question

我有两个表，我想查询2个表以获得报告。

POSITION
+-------------+---------------+
| position_id | position_name |
+-------------+---------------+
|     1       |     E1P1      |
|     2       |     E1P2      |
|     3       |     E3P3      |
|     4       |     E4P4      |
+-------------+---------------+   

PEOPLE
+------------+-------------+--------------------+
| people_id  | people_name | people_position_id |
+------------+-------------+--------------------+
|     1      |    JOHN     |         2          |
|     2      |    MARK     |         4          |
+------------+-------------+--------------------+

查询

SELECT position_id, position_name, people_name FROM position
RIGHT JOIN people ON people_position_id = position_id

当我使用简单查询时，我只会得到匹配的行，如何获得全部？

我想获得此结果

+----+----------+--------+
| ID | POSITION | STATUS |
+----+----------+--------+
| 1  |   E1P1   | Empty  |
| 2  |   E1P2   | JOHN   |
| 3  |   E3P3   | Empty  |
| 4  |   E4P4   | MARK   |
+----+----------+--------+

Answer 1

我会在这里使用左连接：

SELECT
    po.position_id,
    po.position_name,
    COALESCE(pe.people_name, 'EMPTY') AS STATUS
FROM position po
LEFT JOIN people pe
    ON po.position_id = pe.people_position_id;

顺便说一句，您当前的右联接尝试失败的原因是您已将people表放置在联接的右侧。这意味着不匹配的位置记录将被丢弃。这是我上面的答案，是使用右连接重写的：

SELECT
    po.position_id,
    po.position_name,
    COALESCE(pe.people_name, 'EMPTY') AS STATUS
FROM people pe
RIGHT JOIN position po
    ON po.position_id = pe.people_position_id;

请注意，表顺序已切换。在大多数情况下，您会看到人们使用左联接而不是右联接。

返回所有在MySQL查询中无法处理的哇哇

1 个答案: