#左连接
#查询所有部门信息以及该部门员工信息
SELECT did,dname,eid,ename FROM t_department d LEFT JOIN t_employee e
ON d.did=e.dept_id;
SELECT did,dname,eid,ename FROM t_employee e LEFT JOIN t_department d
ON d.did=e.dept_id;
SELECT did,dname,eid,ename FROM t_department d RIGHT JOIN t_employee e
ON d.did=e.dept_id;
UPDATE t_employee SET dept_id=NULL where eid=5;
SELECT did,dname,eid,ename FROM t_department d LEFT JOIN t_employee e
ON d.did=e.dept_id;
WHERE e.dept_id IS NULL;
#查询部门信息，保留没有员工的部门信息
SELECT did,dname,eid,ename FROM t_department d RIGHT JOIN t_employee e
ON d.did=e.dept_id;
WHERE e.dept_id IS NULL;
SELECT did,dname,eid,ename FROM t_employee e LEFT JOIN t_department d
ON d.did=e.dept_id;
WHERE e.dept_id IS NULL;
#查询所有员工信息，以及员工的部门信息
SELECT eid,ename,did,dname FROM t_employee e LEFT JOIN t_department d
ON e.dept_id=d.did;
SELECT eid,ename,did,dname FROM t_department d LEFT JOIN t_employee e
ON e.dept_id=d.did;
#查询员工信息，仅保留没有分配部门的员工
SELECT eid,ename,did,dname FROM t_employee e LEFT JOIN t_department d
ON e.dept_id=d.did
WHERE e.dept_id IS NULL;
#查询员工信息，不保留没有分配部门的员工
SELECT eid,ename,did,dname FROM t_employee e LEFT JOIN t_department d
ON e.dept_id=d.did
WHERE e.dept_id IS NOT NULL;
SELECT id,ename,tid,dname FROM student_info s LEFT JOIN student_test t
ON s.id_id=t.tid
WHERE t.tid IS NOT NULL;
SELECT * FROM student;
SELECT * FROM souce;
#分析 左右外连接
#显示所有学生的测试信息
SELECT st.s_id,name,C,Java FROM student st LEFT JOIN souce s
ON st.s_id=s.s_id;
#显示参加考试的学生的测试信息
SELECT st.s_id,name,C,Java FROM student st RIGHT JOIN souce s
ON st.s_id=s.s_id;
