mysql - 如何连接两个表，保留不满足 JOIN 条件的行？

Question

ticket                      
+----------+--------+
| ticketID | assign |
+----------+--------+
|     1015 | NULL   |
|     1020 | James  |
|     1021 | Nick   |
+----------+--------+

staffinfo
+---------+-------+
| staffID | staff |
+---------+-------+
|       1 | Jane  |
|       2 | James |
|       3 | Nick  |
|       4 | Cole  |
+---------+-------+

SELECT staff,COUNT(*) as count FROM staffinfo,ticket
WHERE ticket.assign = staffinfo.staff
GROUP BY staff

result:
    +-------+-------+
    | staff | count |
    +-------+-------+
    | James |     1 |
    | Nick  |     1 |
    +-------+-------+

工作正常，但事实上我需要这样的东西:

+-------+-------+
| staff | count |
+-------+-------+
| James |     1 |
| Nick  |     1 |
| Jane  |     0 |
| Cole  |     0 |
+-------+-------+

COUNT不统计表中不存在的记录，刚开始学SQL，想问问有没有办法统计成上面的结果？

Answer 1

你应该使用LEFT JOIN

SELECT  a.staff,    COUNT(b.assign) as count 
FROM    staffinfo a
        LEFT JOIN ticket b
            ON b.assign = a.staff
GROUP   BY a.staff

• SQLFiddle Demo

要全面了解联接，请访问以下链接:

• Visual Representation of SQL Joins