MySQL中在含有GROUP BY的SELECT語句中顯示COUNT()為0的結果
mysql> select * from t1;
+-------+------+
| level | name |
+-------+------+
| 1 | a |
| 1 | b |
| 1 | c |
| 2 | a |
| 2 | b |
| 4 | a |
| 5 | b |
+-------+------+
7 rows in set (0.00 sec)
mysql> select * from t2;
+------+
| id |
+------+
| 1 |
| 2 |
| 3 |
| 4 |
| 5 |
+------+
5 rows in set (0.02 sec)
mysql> select t2.id as level,count(case when name is not null then 1 end) as count
-> from t2 left join t1 on t2.id = t1.level
-> group by t2.id;
+-------+-------+
| level | count |
+-------+-------+
| 1 | 3 |
| 2 | 2 |
| 3 | 0 |
| 4 | 1 |
| 5 | 1 |
+-------+-------+
5 rows in set (0.00 sec)
