掌握MySQL之查询语句的基本操作
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mysql视频教程栏目介绍查询语句的操作
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一.查询语句的基本操作
1.查询语句的基本操作 - select - from - where:约束条件 - group by:分组 - having：过滤 - distinct：去重 - order by：排序 - limit：限制查询记录的数量 - 聚合函数: count（计数） max（最大值） min（最小值） avg（平均值） sum(求和)
二.单表查询
1、前期表与数据准备
# 创建一张部门表
create table emp(
id int not null unique auto_increment,
name varchar(20) not null,
sex enum('male','female') not null default 'male', #大部分是男的
age int(3) unsigned not null default 28,
hire_date date not null,
post varchar(50),
post_comment varchar(100),
salary double(15,2),
office int, # 一个部门一个屋子
depart_id int
);
# 插入记录
# 三个部门：教学，销售，运营
insert into emp(name,sex,age,hire_date,post,salary,office,depart_id) values
('tank','male',17,'20170301','张江第一帅形象代言部门',7300.33,401,1), # 以下是教学部
('egon','male',78,'20150302','teacher',1000000.31,401,1),
('kevin','male',81,'20130305','teacher',8300,401,1),
('jason','male',73,'20140701','teacher',3500,401,1),
('owen','male',28,'20121101','teacher',2100,401,1),
('jerry','female',18,'20110211','teacher',9000,401,1),
('大饼','male',18,'19000301','teacher',30000,401,1),
('sean','male',48,'20101111','teacher',10000,401,1),
('歪歪','female',48,'20150311','sale',3000.13,402,2),# 以下是销售部门
('丫丫','female',38,'20101101','sale',2000.35,402,2),
('丁丁','female',18,'20110312','sale',1000.37,402,2),
('星星','female',18,'20160513','sale',3000.29,402,2),
('格格','female',28,'20170127','sale',4000.33,402,2),
('张野','male',28,'20160311','operation',10000.13,403,3), # 以下是运营部门
('程咬金','male',18,'19970312','operation',20000,403,3),
('程咬银','female',18,'20130311','operation',19000,403,3),
('程咬铜','male',18,'20150411','operation',18000,403,3),
('程咬铁','female',18,'20140512','operation',17000,403,3);
# PS：如果在windows系统中，插入中文字符，select的结果为空白，可以将所有字符编码统一设置成gbk
- select * from emp; # 若数据比较多，比较凌乱，可以在表后面+ \G
- select * from emp\G
- select * from emp; # 若数据比较多，比较凌乱，可以在表后面+ \G - select * from emp\G
重点：
写SQL语句必须遵循两点:
- 书写顺序:
# 获取id为 4、5的两条记录
select * from emp where id > 3 and id < 6;
- select
- from
- where
- 执行顺序:
比如: 图书管理员，得先找到是哪一个图书馆(哪张表),
再找这本书在图书馆的哪个位置(哪一条记录), 最后查找这个本书中某一页(哪些字段值);
select * from emp where id > 3 and id < 6;
- from ---> 找到图书馆
- where ---> 找到书的位置
- select ---> 找到书本中的某一页
注意: 必须记住SQL语句的 书写顺序 与 执行顺序(*******);
1.where(约束条件)
# PS: 根据执行顺序来书写 SQL语句，一步一步来写; # 1.查询id大于等于3小于等于6的数据(and:与) select * from emp where id >= 3 and id <= 6; mysql> SELECT * FROM EMP WHERE ID > 3 AND ID <= 6; +----+-------+--------+-----+------------+---------+--------------+---------+--------+-----------+ | id | name | sex | age | hire_date | post | post_comment | salary | office | depart_id | +----+-------+--------+-----+------------+---------+--------------+---------+--------+-----------+ | 4 | jason | male | 73 | 2014-07-01 | teacher | NULL | 3500.00 | 401 | 1 | | 5 | owen | male | 28 | 2012-11-01 | teacher | NULL | 2100.00 | 401 | 1 | | 6 | jerry | female | 18 | 2011-02-11 | teacher | NULL | 9000.00 | 401 | 1 | +----+-------+--------+-----+------------+---------+--------------+---------+--------+-----------+ 3 rows in set (0.00 sec) #可以使用between()and() :两者之间 mysql> select * from emp where id between 3 and 6; +----+-------+--------+-----+------------+---------+--------------+---------+--------+-----------+ | id | name | sex | age | hire_date | post | post_comment | salary | office | depart_id | +----+-------+--------+-----+------------+---------+--------------+---------+--------+-----------+ | 3 | kevin | male | 81 | 2013-03-05 | teacher | NULL | 8300.00 | 401 | 1 | | 4 | jason | male | 73 | 2014-07-01 | teacher | NULL | 3500.00 | 401 | 1 | | 5 | owen | male | 28 | 2012-11-01 | teacher | NULL | 2100.00 | 401 | 1 | | 6 | jerry | female | 18 | 2011-02-11 | teacher | NULL | 9000.00 | 401 | 1 | +----+-------+--------+-----+------------+---------+--------------+---------+--------+-----------+ 4 rows in set (0.00 sec) # 2.查询薪资是20000或者18000或者17000的数据 # or: 或者 select * from emp where salary=20000 or salary=18000 or salary=17000; # in: 在什么里 select * from emp where salary in (20000, 18000, 17000); mysql> select * from emp where salary = 20000 or salary = 18000 or salary = 17000; +----+-----------+--------+-----+------------+-----------+--------------+----------+--------+-----------+ | id | name | sex | age | hire_date | post | post_comment | salary | office | depart_id | +----+-----------+--------+-----+------------+-----------+--------------+----------+--------+-----------+ | 15 | 程咬金 | male | 18 | 1997-03-12 | operation | NULL | 20000.00 | 403 | 3 | | 17 | 程咬铜 | male | 18 | 2015-04-11 | operation | NULL | 18000.00 | 403 | 3 | | 18 | 程咬铁 | female | 18 | 2014-05-12 | operation | NULL | 17000.00 | 403 | 3 | +----+-----------+--------+-----+------------+-----------+--------------+----------+--------+-----------+ 3 rows in set (0.00 sec) # 也可以使用in(3,4,5,6) select * from emp where id in(3,4,5,6); +----+-------+--------+-----+------------+---------+--------------+---------+--------+-----------+ | id | name | sex | age | hire_date | post | post_comment | salary | office | depart_id | +----+-------+--------+-----+------------+---------+--------------+---------+--------+-----------+ | 3 | kevin | male | 81 | 2013-03-05 | teacher | NULL | 8300.00 | 401 | 1 | | 4 | jason | male | 73 | 2014-07-01 | teacher | NULL | 3500.00 | 401 | 1 | | 5 | owen | male | 28 | 2012-11-01 | teacher | NULL | 2100.00 | 401 | 1 | | 6 | jerry | female | 18 | 2011-02-11 | teacher | NULL | 9000.00 | 401 | 1 | +----+-------+--------+-----+------------+---------+--------------+---------+--------+-----------+ 4 rows in set (0.00 sec) # 3.查询员工姓名中包含o字母 的 员工姓名和薪资 # like: 模糊匹配 # %: 匹配0个或多个任意字符(若前后都有%(%o%),查所有带有索引字符的记录,若头有%而尾没有(%o),则查所有尾带有索引字符的记录,若尾有%而头没有(o%),则查所有头带有索引字符的记录,若前后都没有%(o),则只会查询只有索引字符的字符) # _: 匹配一个任意字符 select name, salary from emp where name like "%o%"; +-------+------------+ | name | salary | +-------+------------+ | egon | 1000000.31 | | jason | 3500.00 | | owen | 2100.00 | +-------+------------+ 3 rows in set (0.00 sec) mysql> select name, salary from emp where name like "o"; +------+--------+ | name | salary | +------+--------+ | o | NULL | +------+--------+ 1 row in set (0.00 sec) # 4.查找名字个数为3个的员工 名字 与 薪资 select name, salary from emp where name like "___"; +-----------+----------+ | name | salary | +-----------+----------+ | 程咬金 | 20000.00 | | 程咬银 | 19000.00 | | 程咬铜 | 18000.00 | | 程咬铁 | 17000.00 | +-----------+----------+ 4 rows in set (0.00 sec) # 或者 # char_length(name): 计算名字字符的长度 select name, salary from emp where char_length(name) = 4; mysql> select name, salary from emp where char_length(name) = 3; +-----------+----------+ | name | salary | +-----------+----------+ | 程咬金 | 20000.00 | | 程咬银 | 19000.00 | | 程咬铜 | 18000.00 | | 程咬铁 | 17000.00 | +-----------+----------+ 4 rows in set (0.00 sec) # 5.查询id小于3或者大于6的数据 # not in: 不再什么什么中 select * from emp where id not in (3, 4, 5, 6); select * from emp where id not between 3 and 6; # 6.查询薪资不在20000，18000，17000范围的数据 select * from emp where salary not in (20000, 18000, 17000); # 7.查询岗位描述为空的 员工名 与 岗位名 post_comment # 用等于号无法判定空字段 select name, post from emp where post_comment = null; select * from emp where post_comment = null; Empty set (0.00 sec) # 注意: 针对null的值 需要使用 is select name, post from emp where post_comment is null; select name, post_comment from emp where post_comment is null; +-----------+--------------+ | name | post_comment | +-----------+--------------+ | tank | NULL | | egon | NULL | | kevin | NULL | | jason | NULL | | owen | NULL | | jerry | NULL | | 大饼 | NULL | | sean | NULL | | 歪歪 | NULL | | 丫丫 | NULL | | 丁丁 | NULL | | 星星 | NULL | | 格格 | NULL | | 张野 | NULL | | 程咬金 | NULL | | 程咬银 | NULL | | 程咬铜 | NULL | | 程咬铁 | NULL | | o | NULL | +-----------+--------------+ 19 rows in set (0.00 sec)
2.group by(分组)
- 书写顺序: - select - from - where - group by - 执行顺序: - from - where - group by - select # 什么叫分组 # 比如: 一张员工表中有性别字段，可以根据性别分组，一组是男性，一组是女性，或者是根据部门分组，有教学部、销售部等... # 1.根据部门分组 # 非严格模式下可以获取 分组条件post 以外的字段数据 select post, salary from emp group by post; # 设置严格模式: show variables like "%mode%"; # 全局设置: 永久有效 set global sql_mode="strict_trans_tables,only_full_group_by"; """ 设置sql_mode为only_full_group_by，意味着以后但凡分组，只能取到分组的依据， 不应该在去取组里面的单个元素的值，那样的话分组就没有意义了，并且会报错,因为不分组就是对单个元素信息的随意获取 """ mysql> select post, salary from emp group by post; ERROR 1046 (3D000): No database selected # 可以同聚合函数，间接获取其他字段数据 聚合函数: count: 计数 max: 最大值 min: 最小值 avg: 平均值 sum: 求和 group_concat(name): 可以将分组后的 所有名字获取并进行拼接 # 指定以:拼接 select post, group_concat(name) from emp group by post; select post, group_concat(name, ':') from emp group by post; 2.获取每个 部门 的最高工资 select post,max(salary) from emp group by post; mysql> select post,max(salary) from emp group by post; +-----------------------------------+-------------+ | post | max(salary) | +-----------------------------------+-------------+ | NULL | NULL | | operation | 20000.00 | | sale | 4000.33 | | teacher | 1000000.31 | | 张江第一帅形象代言部门 | 7300.33 | +-----------------------------------+-------------+ 5 rows in set (0.00 sec) 补充: as 别名: 可以给字段 加一个 别名 select post as '部门', max(salary) as '薪资' from emp group by post; # 也可以简写,但是不推荐 select post '部门', max(salary) '薪资' from emp group by post; mysql> select post as '部门', max(salary) as'薪资' -> from emp group by post; +-----------------------------------+------------+ | 部门 | 薪资 | +-----------------------------------+------------+ | NULL | NULL | | operation | 20000.00 | | sale | 4000.33 | | teacher | 1000000.31 | | 张江第一帅形象代言部门 | 7300.33 | +-----------------------------------+------------+ 5 rows in set (0.00 sec) 3.每个部门的最低工资 select post, min(salary) from emp group by post; 4.每个部门的平均工资 select post, avg(salary) from emp group by post; 5.每个部门的工资总和 select post, sum(salary) from emp group by post; 6.每个部门的员工个数 # count(): 括号中可以填任意非空值 select post, count(salary) from emp group by post; select post, count(post_comment) from emp group by post; 查询岗位名以及各岗位内包含的员工个数 select post, count(id) from emp group by post; 查询公司内男员工和女员工的个数 select sex, count(*) from emp group by sex; 查询男员工与男员工的平均薪资，女员工与女员工的平均薪资 select sex, avg(salary) from emp group by sex; .统计各部门年龄在30岁以上的员工平均工资: # 步骤: 先找到表，再找年龄30岁以上，再根据部门分组，最后求平均薪资; select post, avg(salary) from emp where age > 30 group by post;
总结: 聚合函数，必须跟在group by 后面(执行顺序);
# 聚合函数: 若没有group by 分组，默认将查出来的数据当做一个分组, 也能使用; select max(salary) from emp;
# group_concat(name): 可以将分组后的 所有名字获取并进行拼接
# 默认以, 拼接
select post, group_concat(name) from emp group by post;
# 指定以:拼接
select post, group_concat(name, ':') from emp group by post;
#
select post, group_concat('Name: ', name) from emp group by post;
select post, group_coucat(name) from emp group by post;
mysql> select post, group_concat(name) from emp group by post;
+-----------------------------------+------------------------------------------------+
| post | group_concat(name) |
+-----------------------------------+------------------------------------------------+
| NULL | o |
| operation | 程咬铁,程咬铜,程咬银,程咬金,张野 |
| sale | 格格,星星,丁丁,丫丫,歪歪 |
| teacher | sean,大饼,jerry,owen,jason,kevin,egon |
| 张江第一帅形象代言部门 | tank |
+-----------------------------------+------------------------------------------------+
5 rows in set (0.00 sec)
# 给每个部门的员工名字前 + NB_
select post, group_concat('NB_', name) from emp group by post;
# 拼接部门员工名字+薪资
select post, group_concat(name, ":", salary) from emp group by post;
补充:concat
# 4.补充concat（不分组时用）拼接字符串达到更好的显示效果 as语法并用
select concat('Name: ', name) as '名字', concat('Sal: ', salary) as '薪资' from emp;
mysql> select concat('name: ', name) as '名字',concat('sal: ',salary) as '薪资' from emp;
+-----------------+-----------------+
| 名字 | 薪资 |
+-----------------+-----------------+
| name: tank | sal: 7300.33 |
| name: egon | sal: 1000000.31 |
| name: kevin | sal: 8300.00 |
| name: jason | sal: 3500.00 |
| name: owen | sal: 2100.00 |
| name: jerry | sal: 9000.00 |
| name: 大饼 | sal: 30000.00 |
| name: sean | sal: 10000.00 |
| name: 歪歪 | sal: 3000.13 |
| name: 丫丫 | sal: 2000.35 |
| name: 丁丁 | sal: 1000.37 |
| name: 星星 | sal: 3000.29 |
| name: 格格 | sal: 4000.33 |
| name: 张野 | sal: 10000.13 |
| name: 程咬金 | sal: 20000.00 |
| name: 程咬银 | sal: 19000.00 |
| name: 程咬铜 | sal: 18000.00 |
| name: 程咬铁 | sal: 17000.00 |
| name: o | NULL |
+-----------------+-----------------+
19 rows in set (0.00 sec)
3.having(过滤)
1.having与where语法一样，只不过having必需要在group by后使用; 2.where 不能使用聚合函数，但having可以; - 书写顺序: - select - from - where - group by - having - 执行顺序: - from - where - group by - having - select 1、统计各 部门 年龄在30岁以上的员工平均工资，并且保留平均工资大于10000的部门; select post, avg(salary) from emp where age > 30 group by post having avg(salary) > 10000; mysql> select post, avg(salary) from emp where age > 30 group by post having avg(salary) > 10000; +---------+---------------+ | post | avg(salary) | +---------+---------------+ | teacher | 255450.077500 | +---------+---------------+ 1 row in set (0.00 sec)
4.distinct(去重)
# distinct: 去重 - 书写顺序: - select - distinct - from - where - group by - having 执行顺序: - from - where - group by - having - select - distinct # 注意: 查询的字段值必须是重复的才有效，只要有一个字段值是不重复的就没有效果。 # 若所查字段内有不重复的字段记录就不会去重 select distinct id, post from emp; mysql> select distinct id, post from emp; +----+-----------------------------------+ | id | post | +----+-----------------------------------+ | 1 | 张江第一帅形象代言部门 | | 2 | teacher | | 3 | teacher | | 4 | teacher | | 5 | teacher | | 6 | teacher | | 7 | teacher | | 8 | teacher | | 9 | sale | | 10 | sale | | 11 | sale | | 12 | sale | | 13 | sale | | 14 | operation | | 15 | operation | | 16 | operation | | 17 | operation | | 18 | operation | | 19 | NULL | +----+-----------------------------------+ 19 rows in set (0.00 sec) select distinct post from emp; mysql> select distinct post from emp; +-----------------------------------+ | post | +-----------------------------------+ | 张江第一帅形象代言部门 | | teacher | | sale | | operation | | NULL | +-----------------------------------+ 5 rows in set (0.00 sec)
5.order by(排序)
- 书写顺序: - select - from - where - group by - having - order by 执行顺序: - from - where - group by - having - select - order by # 通过select 查出来的数据再进行排序 # order by 默认升序 # asc升序 # desc降序 # 1、根据薪资进行升序 select name, salary from emp order by salary; # 默认升序 mysql> select name, salary from emp order by salary; +-----------+------------+ | name | salary | +-----------+------------+ | o | NULL | | 丁丁 | 1000.37 | | 丫丫 | 2000.35 | | owen | 2100.00 | | 歪歪 | 3000.13 | | 星星 | 3000.29 | | jason | 3500.00 | | 格格 | 4000.33 | | tank | 7300.33 | | kevin | 8300.00 | | jerry | 9000.00 | | sean | 10000.00 | | 张野 | 10000.13 | | 程咬铁 | 17000.00 | | 程咬铜 | 18000.00 | | 程咬银 | 19000.00 | | 程咬金 | 20000.00 | | 大饼 | 30000.00 | | egon | 1000000.31 | +-----------+------------+ 19 rows in set (0.00 sec) 2.select * from emp order by salary asc; # 指定升序 3.select name, salary from emp order by salary desc; # 指定降序 mysql> select name, salary from emp order by salary desc; +-----------+------------+ | name | salary | +-----------+------------+ | egon | 1000000.31 | | 大饼 | 30000.00 | | 程咬金 | 20000.00 | | 程咬银 | 19000.00 | | 程咬铜 | 18000.00 | | 程咬铁 | 17000.00 | | 张野 | 10000.13 | | sean | 10000.00 | | jerry | 9000.00 | | kevin | 8300.00 | | tank | 7300.33 | | 格格 | 4000.33 | | jason | 3500.00 | | 星星 | 3000.29 | | 歪歪 | 3000.13 | | owen | 2100.00 | | 丫丫 | 2000.35 | | 丁丁 | 1000.37 | | o | NULL | +-----------+------------+ 19 rows in set (0.00 sec) # 先按照age升序，再按照salary降序 select age, salary from emp order by age asc, salary desc; # 统计 各部门(分组) 年龄在10岁以上的员工平均工资，并且保留平均工资大于1000的部门，然后对平均工资进行升序序 select post, avg(salary) from emp where age > 10 group by post having avg(salary) > 1000 order by avg(salary);
9.limit(限制查询记录的数量)
书写顺序: - select - from - order by - limit 执行顺序: - from - select - order by - limit # 1、从第一条开始，获取4条记录; select * from emp limit 4; mysql> select * from emp limit 4; +----+-------+------+-----+------------+-----------------------------------+--------------+------------+--------+-----------+ | id | name | sex | age | hire_date | post | post_comment | salary | office | depart_id | +----+-------+------+-----+------------+-----------------------------------+--------------+------------+--------+-----------+ | 1 | tank | male | 17 | 2017-03-01 | 张江第一帅形象代言部门 | NULL | 7300.33 | 401 | 1 | | 2 | egon | male | 78 | 2015-03-02 | teacher | NULL | 1000000.31 | 401 | 1 | | 3 | kevin | male | 81 | 2013-03-05 | teacher | NULL | 8300.00 | 401 | 1 | | 4 | jason | male | 73 | 2014-07-01 | teacher | NULL | 3500.00 | 401 | 1 | +----+-------+------+-----+------------+-----------------------------------+--------------+------------+--------+-----------+ 4 rows in set (0.00 sec) # 2、limit可以有两个参数, 参数1:是限制的开始位置, 参数2:是从开始位置展示的条数; select * from emp limit 0, 4; select * from emp limit 4, 4; mysql> select * from emp limit 4, 4; +----+--------+--------+-----+------------+---------+--------------+----------+--------+-----------+ | id | name | sex | age | hire_date | post | post_comment | salary | office | depart_id | +----+--------+--------+-----+------------+---------+--------------+----------+--------+-----------+ | 5 | owen | male | 28 | 2012-11-01 | teacher | NULL | 2100.00 | 401 | 1 | | 6 | jerry | female | 18 | 2011-02-11 | teacher | NULL | 9000.00 | 401 | 1 | | 7 | 大饼 | male | 18 | 1900-03-01 | teacher | NULL | 30000.00 | 401 | 1 | | 8 | sean | male | 48 | 2010-11-11 | teacher | NULL | 10000.00 | 401 | 1 | +----+--------+--------+-----+------------+---------+--------------+----------+--------+-----------+ 4 rows in set (0.00 sec) # 3、查询工资最高的人的详细信息 select * from emp order by salary limit 1; mysql> select * from emp order by salary limit 1; +----+------+------+-----+------------+------+--------------+--------+--------+-----------+ | id | name | sex | age | hire_date | post | post_comment | salary | office | depart_id | +----+------+------+-----+------------+------+--------------+--------+--------+-----------+ | 19 | o | male | 28 | 0000-00-00 | NULL | NULL | NULL | NULL | NULL | +----+------+------+-----+------------+------+--------------+--------+--------+-----------+ 1 row in set (0.00 sec)
10.正则
# 在编程中，凡是看到reg开头的，基本上都是跟正则有关 # *: 代表 0 或 多个 select * from emp where name regexp '^程.*(金|银|铜|铁)$'; mysql> select * from emp where name regexp '^程.*(金|银|铜|铁)$'; +----+-----------+--------+-----+------------+-----------+--------------+----------+--------+-----------+ | id | name | sex | age | hire_date | post | post_comment | salary | office | depart_id | +----+-----------+--------+-----+------------+-----------+--------------+----------+--------+-----------+ | 15 | 程咬金 | male | 18 | 1997-03-12 | operation | NULL | 20000.00 | 403 | 3 | | 16 | 程咬银 | female | 18 | 2013-03-11 | operation | NULL | 19000.00 | 403 | 3 | | 17 | 程咬铜 | male | 18 | 2015-04-11 | operation | NULL | 18000.00 | 403 | 3 | | 18 | 程咬铁 | female | 18 | 2014-05-12 | operation | NULL | 17000.00 | 403 | 3 | +----+-----------+--------+-----+------------+-----------+--------------+----------+--------+-----------+ 4 rows in set (0.00 sec)
三.多表查询
多表查询
-关联查询
-子查询
创建表与插入数据准备
#建表
create table dep2(
id int,
name varchar(20)
);
create table emp2(
id int primary key auto_increment,
name varchar(20),
sex enum('male','female') not null default 'male',
age int,
dep_id int
);
#插入数据
insert into dep2 values
(200,'技术'),
(201,'人力资源'),
(202,'销售'),
(203,'运营');
insert into emp2(name,sex,age,dep_id) values
('tank','male',17,200),
('egon','female',48,201),
('kevin','male',38,201),
('jason','female',28,202),
('owen','male',18,200),
('sean','female',18,204);
mysql> select * from dep2;
+------+--------------+
| id | name |
+------+--------------+
| 200 | 技术 |
| 201 | 人力资源 |
| 202 | 销售 |
| 203 | 运营 |
+------+--------------+
4 rows in set (0.00 sec)
mysql> select * from emp2;
+----+-------+--------+------+--------+
| id | name | sex | age | dep_id |
+----+-------+--------+------+--------+
| 1 | tank | male | 17 | 200 |
| 2 | egon | female | 48 | 201 |
| 3 | kevin | male | 38 | 201 |
| 4 | jason | female | 28 | 202 |
| 5 | owen | male | 18 | 200 |
| 6 | sean | female | 18 | 204 |
+----+-------+--------+------+--------+
# 指到了如何根据表关系对字段进行拆分，目的是为了更好的管理，表数据都存放在硬盘中，存不是目的，目的是为了取，所以我们将数据从硬盘读到内存中，接下来我们因应该将他们拼成一张表来查询更加合理;
# 注意: 将拆分的表，再拼接到一起进行查询, 可以通过一张表查另一张表的数据;
1.关联查询
# 左表的一条记录与右表的一条记录都对应一遍称之为 --> "笛卡尔积" PS: 百度科普 # 将所有的数据都对应了一遍，虽然不合理但是其中有合理的数据，现在我们需要做的就是找出合理的数据
举例:通过之前准备的数据进行多表操作 1、查询 员工 以及所在 部门 的信息; # 通过where 约束条件 select * from emp2, dep2 where emp2.dep_id = dep2.id; 2、查询 部门 为 技术部 的 员工 及 部门信息 select * from emp2, dep2 where emp2.dep_id = dep2.id and dep2.name = '技术';
联表查询: -内链接:只去两张表有对应关系的记录 select * from emp2 inner join dep2 on emp2.dep_id = dep2.id; mysql> select * from emp2 inner join dep2 on emp2.dep_id = dep2.id; +----+-------+--------+------+--------+------+--------------+ | id | name | sex | age | dep_id | id | name | +----+-------+--------+------+--------+------+--------------+ | 1 | tank | male | 17 | 200 | 200 | 技术 | | 2 | egon | female | 48 | 201 | 201 | 人力资源 | | 3 | kevin | male | 38 | 201 | 201 | 人力资源 | | 4 | jason | female | 28 | 202 | 202 | 销售 | | 5 | owen | male | 18 | 200 | 200 | 技术 | +----+-------+--------+------+--------+------+--------------+ 5 rows in set (0.05 sec) 2、left join # 2、左连接: 在内连接的基础上保留左表没有对应关系的记录(以null补全) select * from emp2 left join dep2 on emp2.dep_id = dep2.id; mysql> select * from emp2 left join dep2 on emp2.dep_id = dep2.id; +----+-------+--------+------+--------+------+--------------+ | id | name | sex | age | dep_id | id | name | +----+-------+--------+------+--------+------+--------------+ | 1 | tank | male | 17 | 200 | 200 | 技术 | | 5 | owen | male | 18 | 200 | 200 | 技术 | | 2 | egon | female | 48 | 201 | 201 | 人力资源 | | 3 | kevin | male | 38 | 201 | 201 | 人力资源 | | 4 | jason | female | 28 | 202 | 202 | 销售 | | 6 | sean | female | 18 | 204 | NULL | NULL | +----+-------+--------+------+--------+------+--------------+ # 3、右连接: 在内连接的基础上保留右表没有对应关系的记录 select * from emp2 right join dep2 on emp2.dep_id = dep2.id; mysql> select * from emp2 right join dep2 on emp2.dep_id = dep2.id; +------+-------+--------+------+--------+------+--------------+ | id | name | sex | age | dep_id | id | name | +------+-------+--------+------+--------+------+--------------+ | 1 | tank | male | 17 | 200 | 200 | 技术 | | 2 | egon | female | 48 | 201 | 201 | 人力资源 | | 3 | kevin | male | 38 | 201 | 201 | 人力资源 | | 4 | jason | female | 28 | 202 | 202 | 销售 | | 5 | owen | male | 18 | 200 | 200 | 技术 | | NULL | NULL | NULL | NULL | NULL | 203 | 运营 | +------+-------+--------+------+--------+------+--------------+ 6 rows in set (0.00 sec) - 全连接: union # 4、全连接: 在内连接的基础上 保留左、右表没有对应关系的记录 select * from emp2 left join dep2 on emp2.dep_id = dep2.id union select * from emp2 right join dep2 on emp2.dep_id = dep2.id;
2.子查询
# 子查询就是将一个查询语句的结果用括号括起来，当做另一个查询语句的条件去用 # 查询部门是技术或者人力资源的员工信息 ''' 先获取技术部和人力资源的id号，再去员工表里根据前面的id筛选出符合要求的员工信息; ''' select * from emp2 where dep_id in (select id from dep2 where name='技术' or name='人力资源'); # 2.每个部门最新入职的员工 思路：先查每个部门最新入职的员工，再按部门对应上联表查询 # 查第一张emp表 # 第四步 select t1.id, t1.name, t1.hire_date, t2.* from # 第二步 # 拼接了 t1（emp） 与 t2（各部门最新入职的员工数据虚拟表）表的数据 emp as t1 inner join # 第一步 # 子查询获取emp表中的 部门名称与最新入职的时间字段值 生成一张虚拟表 (select post, max(hire_date) as max_date from emp group by post) as t2 on # 若 t1.post 的值与 t2.post 条件成立，则拼接对应的记录; t1.post = t2.post # 第三步 where t1.hire_date = t2.max_date;
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