sql进阶：分组查询、子查询、连接查询

SELECT in_dpt, COUNT(in_dpt) AS emp_count -> FROM employee GROUP BY in_dpt;

SELECT in_dpt, COUNT(in_dpt) AS emp_count FROM employee -> GROUP BY in_dpt HAVING in_dpt IN ('dpt3', 'dpt4');

SELECT of_dpt, COUNT(proj_name) AS proj_count FROM project -> GROUP BY of_dpt HAVING of_dpt IN -> (SELECT in_dpt FROM employee WHERE name='Tom');

SELECT id,name,people_num FROM employee JOIN department ON employee.in_dpt = department.dpt_name ORDER BY id;