3.SQL99语法实现多表查询
3.1 内连接(INNER JOIN)的实现
语法
SELECT 字段列表 FROM A表 INNER JOIN B表 ON 关联条件 WHERE 等其他子句;
例
SELECT e.employee_id, e.last_name, e.department_id, d.department_id, d.location_id FROM employees e JOIN departments d ON (e.department_id = d.department_id);
3.2 外连接(OUTER JOIN)的实现
3.2.1.左外连接
语法
#实现查询结果是A SELECT 字段列表 FROM A表 LEFT JOIN B表 ON 关联条件 WHERE 等其他子句; 例 SELECT e.last_name, e.department_id, d.department_name FROM employees e LEFT OUTER JOIN departments d ON (e.department_id = d.department_id) ;
3.2.2.右外连接
SELECT 字段列表 FROM A表 RIGHT JOIN B表 ON 关联条件 WHERE 等其他子句; 例 SELECT e.last_name, e.department_id, d.department_name FROM employees e RIGHT OUTER JOIN departments d ON (e.department_id = d.department_id) ;
3.2.3.满外连接
- 满外连接的结果 = 左右表匹配的数据 + 左表没有匹配到的数据 + 右表没有匹配到的数据。
- SQL99是支持满外连接的。使用FULL JOIN 或 FULL OUTER JOIN来实现。
- 需要注意的是，MySQL不支持FULL JOIN，但是可以用 LEFT JOIN UNION RIGHT join代替。
4.UNION的使用
合并查询结果 利用UNION关键字，可以给出多条SELECT语句，并将它们的结果组合成单个结果集。合并 时，两个表对应的列数和数据类型必须相同，并且相互对应。各个SELECT语句之间使用UNION或UNION ALL关键字分隔
语法
SELECT column,... FROM table1 UNION [ALL] SELECT column,... FROM table2
注意：执行UNION ALL语句时所需要的资源比UNION语句少。如果明确知道合并数据后的结果数据不存在重复数据，或者不需要去除重复的数据，则尽量使用UNION ALL语句，以提高数据查询的效率
。
5种SQL JOINS的实现
代码实现
#中图：内连接 A∩B SELECT employee_id,last_name,department_name FROM employees e JOIN departments d ON e.`department_id` = d.`department_id`;
#左上图：左外连接 SELECT employee_id,last_name,department_name FROM employees e LEFT JOIN departments d ON e.`department_id` = d.`department_id`;
#右上图：右外连接 SELECT employee_id,last_name,department_name FROM employees e RIGHT JOIN departments d ON e.`department_id` = d.`department_id`;
#左中图：A - A∩B SELECT employee_id,last_name,department_name FROM employees e LEFT JOIN departments d ON e.`department_id` = d.`department_id` WHERE d.`department_id` IS NULL
#右中图：B-A∩B SELECT employee_id,last_name,department_name FROM employees e RIGHT JOIN departments d ON e.`department_id` = d.`department_id` WHERE e.`department_id` IS NULL
#右下图 #左中图 + 右中图 A ∪B- A∩B 或者 (A - A∩B) ∪ （B - A∩B） SELECT employee_id,last_name,department_name FROM employees e LEFT JOIN departments d ON e.`department_id` = d.`department_id` WHERE d.`department_id` IS NULL UNION ALL SELECT employee_id,last_name,department_name FROM employees e RIGHT JOIN departments d ON e.`department_id` = d.`department_id` WHERE e.`department_id` IS NULL
左下图 SELECT employee_id,last_name,department_name FROM employees e LEFT JOIN departments d ON e.`department_id` = d.`department_id` WHERE d.`department_id` IS NULL UNION ALL #没有去重操作，效率高 SELECT employee_id,last_name,department_name FROM employees e RIGHT JOIN departments d ON e.`department_id` = d.`department_id`;
