SQL主要有两个标准： SQL92和 SQL99。
一般来说，SQL92的形式更简单，但SQL语句的可读性较差；SQL99相比于SQL92，语法更加复杂，但可读性更强。

SQL99使用JOIN...ON...实现多表查询。

1. 内连接， JOIN...ON...，同INNER JOIN...ON...
2. 左外连接， LEFT JOIN...ON...，同LEFT OUTER JOIN...ON...
3. 右外连接，RIGHT JOIN...ON...，同RIGHT OUTER JOIN...ON...

SQL99下的内连接

1. 查询员工的id及其所在部门名称。JOIN...ON...。
   INNER JOIN...ON...，可简写为JOIN...ON...

# 查询员工的id及其所在部门名称
# JOIN...ON...
SELECT employee_id,department_name
FROM employees e JOIN departments d
ON e.department_id = d.department_id; -- 返回106行记录。
# INNER JOIN...ON...
SELECT employee_id,department_name
FROM employees e INNER JOIN departments d
ON e.department_id = d.department_id; -- 返回106行记录。
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2. 查询员工的id、所在部门名称、所在城市。JOIN...ON...JOIN...ON...或者JOIN ... JOIN ...ON ...AND...

# JOIN...ON...JOIN...ON...
SELECT employee_id,department_name,city
FROM employees e JOIN departments d
ON e.department_id = d.department_id
JOIN locations l
ON d.location_id = l.location_id; -- 返回106行记录

# JOIN ... JOIN ...ON ...AND...
SELECT e.last_name,d.department_name,l.city
FROM employees e JOIN departments d JOIN locations l
ON e.department_id = d.department_id
AND d.location_id = l.location_id; -- 返回106行记录
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SQL99下的外连接

左外连接

查询所有员工的id及其所在部门名称。LEFT JOIN...ON...，左外连接。
LEFT OUTER JOIN...ON...，可简写为LEFT JOIN...ON...。

# LEFT JOIN...ON
SELECT employee_id,department_name
FROM employees e LEFT JOIN departments d
ON e.department_id = d.department_id; -- 返回107行记录

# LEFT OUTER JOIN...ON
SELECT employee_id,department_name
FROM employees e LEFT OUTER JOIN departments d
ON e.department_id = d.department_id; -- 返回107行记录
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右外连接

查询所有部门及其所有员工的id。RIGHT JOIN...ON...，右外连接。
RIGHT OUTER JOIN...ON...，可简写为RIGHT JOIN...ON...。

SELECT employee_id,department_name
FROM employees e RIGHT JOIN departments d
ON e.department_id = d.department_id;-- 返回122行记录
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满外连接

SELECT employee_id,department_name
FROM employees e FULL JOIN departments d
ON e.department_id = d.department_id;
/*
报错：
You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'FULL JOIN departments d
ON e.department_id = d.department_id' at line 2
*/
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SQL99下满外连接，使用FULL JOIN ...ON，但MySQL不支持SQL99满外连接的语法，Orcale支持这种语法。