通常，having 和 where 子句在没有 group by 子句时有什么区别吗(特别是在 MySQL、PostrgrSQL、Oracle 中和 MSSQL)？
select * from user where foo > 3
似乎等同于
从拥有 foo > 3 的用户中选择 *
最佳答案
至少有一个巨大的区别:
HAVING 子句将不使用索引:
explain select username from users having user_id = 32654456;
+----+-------------+--------+-------+---------------+---------+---------+------+--------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------+-------+---------------+---------+---------+------+--------+-------------+
| 1 | SIMPLE | users | index | NULL | PRIMARY | 32 | NULL | 661107 | Using index |
+----+-------------+--------+-------+---------------+---------+---------+------+--------+-------------+
WHERE 子句将:
explain select username from users where user_id = 32654456;
+----+-------------+--------+-------+---------------+---------+---------+-------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------+-------+---------------+---------+---------+-------+------+-------------+
| 1 | SIMPLE | users | const | PRIMARY | PRIMARY | 32 | const | 1 | Using index |
+----+-------------+--------+-------+---------------+---------+---------+-------+------+-------------+
关于mysql - 当没有 "having"子句时， "where"和 "group by"是否完全等价？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/36963386/