【MySQL8】内外连接的SQL99 和 SQL92的两种写法

我先说个结论

• MySQL仅支持SQL92语法的内连接，不支持SQL92语法的外连接
• MySQL支持SQL99语法的内连接和外连接
• 所以，一了百了，内连接和外连接的写法全写SQL99语法好了
• 再补一句，SQL99支持满外连接，但是MySQL不支持FULL JOIN（满外连接），但是可以用 LEFT JOIN UNION RIGHT join代替

1.内连接

#SQL92语法实现内连接：
SELECT employee_id,department_name
FROM employees e,departments d
WHERE e.`department_id` = d.department_id;   # 需要使用左外连接


#SQL99语法实现内连接：
SELECT last_name,department_name
FROM employees e INNER JOIN departments d
ON e.`department_id` = d.`department_id`;

SELECT last_name,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`;

2.外连接

#SQL92语法实现外连接：使用 +  ----------MySQL不支持SQL92语法中外连接的写法！
#不支持：
SELECT employee_id,department_name
FROM employees e,departments d
WHERE e.`department_id` = d.department_id(+);

#SQL99语法中使用 JOIN ...ON 的方式实现多表的查询。这种方式也能解决外连接的问题。MySQL是支持此种方式的。
#SQL99语法如何实现多表的查询。


#SQL99语法实现外连接：

#练习：查询所有的员工的last_name,department_name信息 
# 左外连接：
SELECT last_name,department_name
FROM employees e LEFT JOIN departments d
ON e.`department_id` = d.`department_id`;

#右外连接：
SELECT last_name,department_name
FROM employees e RIGHT OUTER JOIN departments d
ON e.`department_id` = d.`department_id`;


#满外连接：mysql不支持FULL OUTER JOIN
SELECT last_name,department_name
FROM employees e FULL OUTER JOIN departments d
ON e.`department_id` = d.`department_id`;