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语法:
select 查询列表 from 表1 别名 【连接类型】 join 表2 别名 on 连接条件 【where 筛选条件】 【group by 分组】 【having 筛选条件】 【order by 排序列表】
分类:
#一)内连接/*语法:select 查询列表from 表1 别名inner join 表2 别名on 连接条件;分类:等值非等值自连接特点:①添加排序、分组、筛选②inner可以省略③ 筛选条件放在where后面,连接条件放在on后面,提高分离性,便于阅读④inner join连接和sql92语法中的等值连接效果是一样的,都是查询多表的交集
等值连接:
#1、等值连接#案例1.查询员工名、部门名SELECT last_name,department_nameFROM departments d JOIN employees eON e.`department_id` = d.`department_id`;#案例2.查询名字中包含e的员工名和工种名(添加筛选)SELECT last_name,job_titleFROM employees eINNER JOIN jobs jON e.`job_id`= j.`job_id`WHERE e.`last_name` LIKE '%e%';#3. 查询部门个数>3的城市名和部门个数,(添加分组+筛选)#①查询每个城市的部门个数#②在①结果上筛选满足条件的SELECT city,COUNT(*) 部门个数FROM departments dINNER JOIN locations lON d.`location_id`=l.`location_id`GROUP BY cityHAVING COUNT(*)>3;#案例4.查询哪个部门的员工个数>3的部门名和员工个数,并按个数降序(添加排序)#①查询每个部门的员工个数SELECT COUNT(*),department_nameFROM employees eINNER JOIN departments dON e.`department_id`=d.`department_id`GROUP BY department_name#② 在①结果上筛选员工个数>3的记录,并排序SELECT COUNT(*) 个数,department_nameFROM employees eINNER JOIN departments dON e.`department_id`=d.`department_id`GROUP BY department_nameHAVING COUNT(*)>3ORDER BY COUNT(*) DESC;#5.查询员工名、部门名、工种名,并按部门名降序(添加三表连接)SELECT last_name,department_name,job_titleFROM employees eINNER JOIN departments d ON e.`department_id`=d.`department_id`INNER JOIN jobs j ON e.`job_id` = j.`job_id`ORDER BY department_name DESC;
非等值连接:
#二)非等值连接#查询员工的工资级别SELECT salary,grade_levelFROM employees e JOIN job_grades g ON e.`salary` BETWEEN g.`lowest_sal` AND g.`highest_sal`; #查询工资级别的个数>20的个数,并且按工资级别降序 SELECT COUNT(*),grade_levelFROM employees e JOIN job_grades g ON e.`salary` BETWEEN g.`lowest_sal` AND g.`highest_sal` GROUP BY grade_level HAVING COUNT(*)>20 ORDER BY grade_level DESC;
自连接:
#三)自连接 #查询员工的名字、上级的名字 SELECT e.last_name,m.last_name FROM employees e JOIN employees m ON e.`manager_id`= m.`employee_id`; #查询姓名中包含字符k的员工的名字、上级的名字 SELECT e.last_name,m.last_name FROM employees e JOIN employees m ON e.`manager_id`= m.`employee_id` WHERE e.`last_name` LIKE '%k%';
/* 应用场景:用于查询一个表中有,另一个表没有的记录 特点: 1、外连接的查询结果为主表中的所有记录 如果从表中有和它匹配的,则显示匹配的值 如果从表中没有和它匹配的,则显示null 外连接查询结果=内连接结果+主表中有而从表没有的记录 2、左外连接,left join左边的是主表 右外连接,right join右边的是主表 3、左外和右外交换两个表的顺序,可以实现同样的效果 4、全外连接=内连接的结果+表1中有但表2没有的+表2中有但表1没有的 */ #引入:查询男朋友 不在男神表的的女神名 SELECT * FROM beauty; SELECT * FROM boys; #左外连接 SELECT b.*,bo.* FROM boys bo LEFT OUTER JOIN beauty b ON b.`boyfriend_id` = bo.`id` WHERE b.`id` IS NULL; #案例1:查询哪个部门没有员工 #左外 SELECT d.*,e.employee_id FROM departments d LEFT OUTER JOIN employees e ON d.`department_id` = e.`department_id` WHERE e.`employee_id` IS NULL; #右外 SELECT d.*,e.employee_id FROM employees e RIGHT OUTER JOIN departments d ON d.`department_id` = e.`department_id` WHERE e.`employee_id` IS NULL; #全外 USE girls; SELECT b.*,bo.* FROM beauty b FULL OUTER JOIN boys bo ON b.`boyfriend_id` = bo.id; #交叉连接 SELECT b.*,bo.* FROM beauty b CROSS JOIN boys bo; #sql92和 sql99pk /* 功能:sql99支持的较多 可读性:sql99实现连接条件和筛选条件的分离,可读性较高 */
#一、查询编号>3的女神的男朋友信息,如果有则列出详细,如果没有,用null填充SELECT b.id,b.name,bo.*FROM beauty bLEFT OUTER JOIN boys boON b.`boyfriend_id` = bo.`id`WHERE b.`id`>3;#二、查询哪个城市没有部门SELECT cityFROM departments dRIGHT OUTER JOIN locations l ON d.`location_id`=l.`location_id`WHERE d.`department_id` IS NULL;#三、查询部门名为SAL或IT的员工信息SELECT e.*,d.department_name,d.`department_id`FROM departments dLEFT JOIN employees eON d.`department_id` = e.`department_id`WHERE d.`department_name` IN('SAL','IT');SELECT * FROM departmentsWHERE `department_name` IN('SAL','IT');
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