第38章 子查询优化——子查询转换为连接 - xiaoboluo768/qianjinliangfang GitHub Wiki
38.2 优化方案
select count(*) from employees as a,(select distinct emp_no from dept_emp where dept_no = 'd007')b where a.emp_no = b.emp_no;
mysql> select count(*) from employees as a where exists (select emp_no from dept_emp b where a.emp_no = b.emp_no and b.dept_no = 'd007');
......
1 row in set (0.44 sec)
mysql> select count(*) from employees as a,(select distinct emp_no from dept_emp where dept_no = 'd007')b where a.emp_no = b.emp_no;
......
1 row in set (0.07 sec)
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