#第六章、多表查询 #1、熟悉教学常见的几个表 DESC employees; DESC departments; DESC locations; #查询员工名为'Abel'的人在哪个城市工作? SELECT * FROM employees WHERE last_name = 'Abel'; SELECT * FROM departments WHERE department_id = 80; SELECT * FROM locations WHERE location_id = 2500; #2、出现笛卡尔积的错误 #错误的原因:缺少了多表的连接条件 #错误的实现方式:每个员工都与每个部门匹配了一遍 SELECT employee_id,department_name FROM employees,departments; #查询出2889条记录 #错误的方式 SELECT employee_id,department_name FROM employees CROSS JOIN departments; #查询出2889条记录 SELECT * FROM employees; SELECT 2889/107 FROM DUAL; SELECT * FROM departments; #3、多表查询的正确方式:需要有连接条件 SELECT employee_id,department_name FROM employees,departments #两个表的连接条件 WHERE employees.`department_id` = departments.department_id; #4、如果查询语句中出现了多个表中都存在的字段,则必须指明此字段所在的表。 SELECT employees.employee_id,departments.department_name,employees.department_id FROM employees,departments WHERE employees.`department_id` = departments.`department_id`; #建议:从SQL优化的角度,建议多表查询时,每个字段前都指明其所在的表 #5、可以给表起别名,在SELECT和WHERE中使用表的别名 SELECT emp.employee_id,dept.department_name,emp.department_id FROM employees emp,departments dept WHERE emp.`department_id` = dept.department_id; #如果给表起了别名,一旦在SELECT或WHERE中使用表名的话,则必须使用表的别名,而不能再使用表的原名。 #如下的操作是错误的: SELECT emp.employee_id,departments.department_name,emp.department_id FROM employees emp,departments dept WHERE emp.`department_id` = departments.department_id; #6、结论:如果有n个表实现多表的查询,则需要至少n-1个连接条件 #练习:查询员工的employee_id,last_name,department_name,city SELECT e.employee_id,e.last_name,d.department_name,l.city,e.department_id,l.location_id FROM employees e,departments d,locations l WHERE e.`department_id` = d.`department_id` AND d.`location_id` = l.`location_id`; /* 演绎式:提出问题1 ---> 解决问题1 ---> 提出问题2 ---> 解决问题2 ... 归纳式:总 -- 分 */ #7、多表查询的分类 /* 角度1:等值连接 vs 非等值连接 角度2:自连接 vs 非自连接 角度3:内连接 vs 外连接 */ #7.1、等值连接 vs 非等值连接 #非等值连接的例子: SELECT * FROM job_grades; SELECT e.last_name,e.salary,j.grade_level FROM employees e,job_grades j #WHERE e.`salary` BETWEEN j.`lowest_sal` AND j.`highest_sal`; WHERE e.`salary` >= j.`lowest_sal` AND e.`salary` <= j.`highest_sal`; #7.2、自连接 vs 非自连接 #自连接的例子: #练习:查询员工id,员工姓名及其管理者的id和姓名 SELECT emp.employee_id,emp.last_name,mgr.employee_id,mgr.last_name FROM employees emp,employees mgr WHERE emp.`manager_id` = mgr.`employee_id`; #7.3、内连接 vs 外连接 #内连接:合并具有同一列的两个以上的表的行,结果集中不包含一个表与另一个表不匹配的行 SELECT employee_id,department_name FROM employees e,departments d WHERE e.`department_id` = d.`department_id`; #只有106条记录 #外连接:合并具有同一列的两个以上的表的行,结果集中除了包含一个表与另一个表匹配的行之外,还查询到了左表或右表中不匹配的行 #外连接的分类:左外连接、右外连接、满外连接 #左外连接:两个表在连接过程中除了返回满足连接条件的行以外,还返回左表中不满足条件的行,这种连接称为左外连接。 #右外连接:两个表在连接过程中除了返回满足连接条件的行以外,还返回右表中不满足条件的行,这种连接称为右外连接。 #练习:查询所有的员工的last_name,department_name信息 #题目中包含了“所有的”,所以就可能存在不匹配的数据,因此一定是外连接 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 #SQL92语法实现内连接:(上面的写法就是) #SQL92语法实现外连接:使用+ -----MySQL不支持SQL92语法中外连接的写法!(Oracle支持) #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语法实现内连接:(INNER可以省略) SELECT last_name,department_name #FROM employees e INNER JOIN departments d FROM employees e 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; #练习:查询所有的员工的last_name,department_name信息 #左外连接:(因为加了LEFT,所以OUTER也可以省略) SELECT last_name,department_name #FROM employees e LEFT OUTER JOIN departments d FROM employees e LEFT JOIN departments d ON e.department_id = d.department_id; #右外连接:(因为加了RIGHT,所以OUTER也可以省略) SELECT last_name,department_name #FROM employees e RIGHT OUTER JOIN departments d FROM employees e RIGHT JOIN departments d ON e.department_id = d.department_id; #满外连接:MySQL不支持FULL OUTER JOIN的方式(Oracle支持) SELECT last_name,department_name FROM employees e FULL OUTER JOIN departments d ON e.department_id = d.department_id; #8、UNION和UNION ALL的使用 #UNION:会执行去重的操作 #UNION ALL:不会执行去重的操作 #结论: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。 #9、7种JOIN的实现 #中图:内连接 SELECT employee_id,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,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,department_name FROM employees e RIGHT JOIN departments d ON e.department_id = d.department_id; #左中图: SELECT employee_id,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 SELECT employee_id,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:满外连接 #方式1:左上图 UNION ALL 右中图 SELECT employee_id,department_name FROM employees e LEFT JOIN departments d ON e.department_id = d.department_id UNION ALL SELECT employee_id,department_name FROM employees e RIGHT JOIN departments d ON e.department_id = d.department_id WHERE e.department_id IS NULL; #方式2:左中图 UNION ALL 右上图 SELECT employee_id,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,department_name FROM employees e RIGHT JOIN departments d ON e.department_id = d.department_id; #右下图:左中图 UNION ALL 右中图 SELECT employee_id,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,department_name FROM employees e RIGHT JOIN departments d ON e.department_id = d.department_id WHERE e.department_id IS NULL; #10、SQL99语法的新特性1:自然连接 SELECT employee_id,last_name,department_name FROM employees e JOIN departments d ON e.department_id = d.department_id AND e.manager_id = d.manager_id; #NATURAL JOIN:它会帮你自动查询两张连接表中“所有相同的字段”,然后进行“等值连接” SELECT employee_id,last_name,department_name FROM employees e NATURAL JOIN departments d; #11、SQL99语法的新特性2:USING SELECT employee_id,last_name,department_name FROM employees e JOIN departments d ON e.department_id = d.department_id; SELECT employee_id,last_name,department_name FROM employees e JOIN departments d USING (department_id); #拓展 SELECT last_name,job_title,department_name FROM employees INNER JOIN departments INNER JOIN jobs ON employees.department_id = departments.department_id AND employees.job_id = jobs.job_id;