2.4.2    Equi-Joins 对 Non-Equi-Joins
在 Join 查询中如果使用等号操作符把两个表关联起来则为 Equi-Joins, 不是用等号操作符则为 Non-Equi-Joins
Equi-Joins:

 

SELECT s.name supplier_name, p.name part_name

 

FROM supplier s JOIN part p

 

ON s.supplier_id = p.supplier_id;

 

 

Non-Equi-Joins:

SELECT p.name part_name, c.inv_class inv_class

 

FROM part p JOIN inventory_class c

 

ON p.unit_cost BETWEEN c.low_cost AND c.high_cost;
2.4.3    Self Joins

就是一个表的行与该表的另外的行关联

 

1) SELECT e.lname employee, m.lname manager

 

FROM employee e JOIN employee m

 

ON e.manager_emp_id = m.emp_id;

 

Self outer joins:

 

SELECT e.lname employee, m.lname manager

 

FROM employee e LEFT OUTER JOIN employee m

 

ON e.manager_emp_id = m.emp_id;

 

Self non-equi-joins

1) SELECT d1.name team1, d2.name team2

 

FROM department d1 JOIN department d2

 

ON d1.dept_id != d2.dept_id;

 

2) SELECT d1.name team1, d2.name team2

 

FROM department d1 JOIN department d2

 

ON D1.DEPT_ID < D2.DEPT_ID;