MySQL高级SQL语句 常用查询
对 MySQL 数据库的查询，除了基本的查询外，有时候需要对查询的结果集进行处理。
1 按关键字排序
以使用 ORDERBY 语句来完成排序，并最终将排序后的结果返回给用户。这个语句的排序不光可以针对某一个字段，也可以针对多个字段。
SELECT column1,column2,... FROM table_name ORDER BY column1,column2,...
ASC|DESC;
其中ASC代表升序、DESC代表降序
1.1 单字段排序
在数据库中新建一个数据库进行实验操作
mysql> create table info(id int(4) not null primary key auto_increment,name char(10) not null,score decimal(5,2),address varchar(50) default '未知')engine=innodb;
Query OK, 0 rows affected (0.01 sec)
mysql> desc info;
+---------+--------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+---------+--------------+------+-----+---------+----------------+
| id | int(4) | NO | PRI | NULL | auto_increment |
| name | char(10) | NO | | NULL | |
| score | decimal(5,2) | YES | | NULL | |
| address | varchar(50) | YES | | 未知 | |
+---------+--------------+------+-----+---------+----------------+
4 rows in set (0.00 sec)
mysql> insert into info (name,score,address) values ('zhangsan',80,'beijing'),('lisi',90,'nanjing'),('wangwu',70,'beijing'),('zhaoliu',60,'nanjing');
Query OK, 4 rows affected (0.00 sec)
Records: 4 Duplicates: 0 Warnings: 0
mysql> select * from info;
+----+----------+-------+---------+
| id | name | score | address |
+----+----------+-------+---------+
| 1 | zhangsan | 80.00 | beijing |
| 2 | lisi | 90.00 | nanjing |
| 3 | wangwu | 70.00 | beijing |
| 4 | zhaoliu | 60.00 | nanjing |
+----+----------+-------+---------+
4 rows in set (0.00 sec)
mysql> select name,score from info where score>=70 order by score; ##如果不加默认为升序，也就是ASC
+----------+-------+
| name | score |
+----------+-------+
| wangwu | 70.00 |
| zhangsan | 80.00 |
| lisi | 90.00 |
+----------+-------+
3 rows in set (0.00 sec)
mysql> select name,score from info where score>=70 order by score desc; ##降序排序
+----------+-------+
| name | score |
+----------+-------+
| lisi | 90.00 |
| zhangsan | 80.00 |
| wangwu | 70.00 |
+----------+-------+
3 rows in set (0.00 sec)
1.2 多字段排序
重新插入数据
mysql> insert into info (name,score,address) values ('tianqi',80,'beijing'),('heiba',70,'nanjing');
Query OK, 2 rows affected (0.00 sec)
Records: 2 Duplicates: 0 Warnings: 0
mysql> select * from info;
+----+----------+-------+---------+
| id | name | score | address |
+----+----------+-------+---------+
| 1 | zhangsan | 80.00 | beijing |
| 2 | lisi | 90.00 | nanjing |
| 3 | wangwu | 70.00 | beijing |
| 4 | zhaoliu | 60.00 | nanjing |
| 5 | tianqi | 80.00 | beijing |
| 6 | heiba | 70.00 | nanjing |
+----+----------+-------+---------+
6 rows in set (0.00 sec)
mysql> select id,name,score from info where score>=70 order by score desc;
+----+----------+-------+
| id | name | score |
+----+----------+-------+
| 2 | lisi | 90.00 |
| 1 | zhangsan | 80.00 |
| 5 | tianqi | 80.00 |
| 3 | wangwu | 70.00 |
| 6 | heiba | 70.00 |
+----+----------+-------+
5 rows in set (0.00 sec)
ORDER BY 后面跟多个字段时，字段之间使用英文逗号隔开，优先级是按先后顺序而定
mysql> update info set score=80 where id=2;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> select id,name,score from info where score>=70 order by score desc,id desc;
+----+----------+-------+
| id | name | score |
+----+----------+-------+
| 5 | tianqi | 80.00 |
| 2 | lisi | 80.00 |
| 1 | zhangsan | 80.00 |
| 6 | heiba | 70.00 |
| 3 | wangwu | 70.00 |
+----+----------+-------+
5 rows in set (0.00 sec)
2 对结果进行分组
GROUPBY从字面上看， 是以BY后面的内容对查询出的数据进行分组，就是将一个“数 据集”划分成若干个“小区域”，然后针对这些个“小区域”进行数据处理。
GROUPBY 通常都是结合聚合函数一起使用的，常用的聚合函数包括：计数（COUNT）、 求和（SUM）、求平均数（AVG）、最大值（MAX）、最小值（MIN）GROUP BY 分组的时候可以按一个或多个字段对结果进行分组处理。
语法结构
SELECT column_name,aggregate_function(column_name) FROM table_name WHERE column_name operator value GROUP BY column_name;
把分数相同的分组，统计分数相同的人数
mysql> select count(name),score from info group by score;
+-------------+-------+
| count(name) | score |
+-------------+-------+
| 1 | 60.00 |
| 2 | 70.00 |
| 3 | 80.00 |
+-------------+-------+
3 rows in set (0.00 sec)
分数大于70分的进行分组，并按分数降序排序
mysql> select count(name),score from info where score>=70 group by score desc;
+-------------+-------+
| count(name) | score |
+-------------+-------+
| 3 | 80.00 |
| 2 | 70.00 |
+-------------+-------+
2 rows in set (0.00 sec)
求平均成绩
mysql> select avg(score) from info;
+------------+
| avg(score) |
+------------+
| 73.333333 |
+------------+
1 row in set (0.00 sec)
分数在60分以上，按分数分组，按同分数的人数倒叙排序
mysql> select count(name),score from info where score>=60 group by score order by count(name) desc;
+-------------+-------+
| count(name) | score |
+-------------+-------+
| 3 | 80.00 |
| 2 | 70.00 |
| 1 | 60.00 |
+-------------+-------+
3 rows in set (0.00 sec)
3 限制结果条目
在使用 MySQLSELECT 语句进行查询时，结果集返回的是所有匹配的记录。有时候仅需要返回第一行或者前几行，这时候就需要用到 LIMIT 子句。
SELECT column1,column2,...FROM table_name LIMIT [offset,]number
LIMIT 的第一个参数是位置偏移量（可选参数），是设置 MySQL 从哪一行开始显示。如果不设定第一个参数，将会从表中的第一条记录开始显示。需要注意的是，第一条记录的位置偏移量是 0，第二条是 1，以此类推。第二个参数是设置返回记录行的最大数目。
mysql> select id,name,score from info limit 2;
+----+----------+-------+
| id | name | score |
+----+----------+-------+
| 1 | zhangsan | 80.00 |
| 2 | lisi | 80.00 |
+----+----------+-------+
2 rows in set (0.00 sec)
mysql> select id,name,score from info limit 3;
+----+----------+-------+
| id | name | score |
+----+----------+-------+
| 1 | zhangsan | 80.00 |
| 2 | lisi | 80.00 |
| 3 | wangwu | 70.00 |
+----+----------+-------+
3 rows in set (0.00 sec)
mysql> select id,name,score from info limit 1,3;
+----+---------+-------+
| id | name | score |
+----+---------+-------+
| 2 | lisi | 80.00 |
| 3 | wangwu | 70.00 |
| 4 | zhaoliu | 60.00 |
+----+---------+-------+
3 rows in set (0.00 sec)
mysql> select id,name,score from info limit 0,3;
+----+----------+-------+
| id | name | score |
+----+----------+-------+
| 1 | zhangsan | 80.00 |
| 2 | lisi | 80.00 |
| 3 | wangwu | 70.00 |
+----+----------+-------+
3 rows in set (0.00 sec)
limit与order by连用
mysql> select * from info order by score limit 3;
+----+---------+-------+---------+
| id | name | score | address |
+----+---------+-------+---------+
| 4 | zhaoliu | 60.00 | nanjing |
| 6 | heiba | 70.00 | nanjing |
| 3 | wangwu | 70.00 | beijing |
+----+---------+-------+---------+
3 rows in set (0.00 sec)
mysql> select * from info order by score desc limit 3;
+----+----------+-------+---------+
| id | name | score | address |
+----+----------+-------+---------+
| 1 | zhangsan | 80.00 | beijing |
| 2 | lisi | 80.00 | nanjing |
| 5 | tianqi | 80.00 | beijing |
+----+----------+-------+---------+
3 rows in set (0.00 sec)
4 设置别名
在 MySQL 查询时，当表的名字比较长或者表内某些字段比较长时，为了方便书写或者多次使用相同的表，可以给字段列或表设置别名。使用的时候直接使用别名，设置别名使用 AS 语句。
列的别名语法结构：
SELECT column_name AS alias_name FROM table_name;
表的别名语法结构：
SELECT column_name(s) FROM table_name AS alias_name;
在使用 AS 后，可以用 alias_name 代替 table_name，其中 AS 语句是可选的。AS 之后的别名，主要是为表内的列或者表提供临时的名称，在查询过程中使用，库内实际的表名或字段名是不会被改变的。
修改列名
mysql> select count(*) from info;
+----------+
| count(*) |
+----------+
| 6 |
+----------+
1 row in set (0.00 sec)
mysql> select count(*) as '总数' from info;
+--------+
| 总数 |
+--------+
| 6 |
+--------+
1 row in set (0.00 sec)
不用as也同样可以修改列名
mysql> select count(*) '总数' from info;
+--------+
| 总数 |
+--------+
| 6 |
+--------+
1 row in set (0.00 sec)
当表明较长的时候，可以使用 AS 给表设置别名，在查询的过程中直接使用别名
临时设置info的别名为i
mysql> select id,name from info as i;
+----+----------+
| id | name |
+----+----------+
| 1 | zhangsan |
| 2 | lisi |
| 3 | wangwu |
| 4 | zhaoliu |
| 5 | tianqi |
| 6 | heiba |
+----+----------+
6 rows in set (0.00 sec)
mysql> select id,name from info i;
+----+----------+
| id | name |
+----+----------+
| 1 | zhangsan |
| 2 | lisi |
| 3 | wangwu |
| 4 | zhaoliu |
| 5 | tianqi |
| 6 | heiba |
+----+----------+
6 rows in set (0.00 sec)
AS还可以用作连接语句的操作符，比如创建一个表，实现用一条 SQL 语句完成在创建表 tmp 的时候将info表内的数据写入 tmp 表。
mysql> create table tmp as select * from info;
Query OK, 6 rows affected (0.01 sec)
Records: 6 Duplicates: 0 Warnings: 0
mysql> select * from tmp;
+----+----------+-------+---------+
| id | name | score | address |
+----+----------+-------+---------+
| 1 | zhangsan | 80.00 | beijing |
| 2 | lisi | 80.00 | nanjing |
| 3 | wangwu | 70.00 | beijing |
| 4 | zhaoliu | 60.00 | nanjing |
| 5 | tianqi | 80.00 | beijing |
| 6 | heiba | 70.00 | nanjing |
+----+----------+-------+---------+
6 rows in set (0.00 sec)
- 在为表设置别名时，要保证别名不能与数据库中的其他表的名称冲突。
- 列的别名是在结果中有显示的，而表的别名在结果中没有显示，只在执行查询时使用。
5 通配符
通配符主要用于替换字符串中的部分字符，通过部分字符的匹配将相关结果查询出来。 通常通配符都是跟 LIKE 一起使用的，并协同 WHERE 子句共同来完成查询任务。
常用的通配符：
- %：百分号表示零个、一个或多个字符
- _：下划线表示单个字符
筛选除info表中name字段以z开头的
mysql> select * from info where name like 'z%';
+----+----------+-------+---------+
| id | name | score | address |
+----+----------+-------+---------+
| 1 | zhangsan | 80.00 | beijing |
| 4 | zhaoliu | 60.00 | nanjing |
+----+----------+-------+---------+
2 rows in set (0.00 sec)
查看info表中name字段zha和liu中间有任意字符的记录
mysql> select * from info where name like 'zha_liu';
+----+---------+-------+---------+
| id | name | score | address |
+----+---------+-------+---------+
| 4 | zhaoliu | 60.00 | nanjing |
+----+---------+-------+---------+
1 row in set (0.00 sec)
查找l开头，后面有四个字符的数据
mysql> select * from info where name like 'l___';
+----+------+-------+---------+
| id | name | score | address |
+----+------+-------+---------+
| 2 | lisi | 80.00 | nanjing |
+----+------+-------+---------+
1 row in set (0.00 sec)
两种通配符配合使用的
mysql> select * from info where name like '%w_';
+----+--------+-------+---------+
| id | name | score | address |
+----+--------+-------+---------+
| 3 | wangwu | 70.00 | beijing |
+----+--------+-------+---------+
1 row in set (0.00 sec)
6 子查询
子查询也被称作内查询或者嵌套查询，是指在一个查询语句里面还嵌套着另一个查询语句。子查询语句是先于主查询语句被执行的，其结果作为外层的条件返回给主查询进行下一 步的查询过滤。子查询不仅可以在 SELECT 语句中使用，在 INERT、UPDATE、DELETE中也同样适用。在嵌套的时候，子查询内部还可以再次嵌套新的子查询，也就是说可以多层嵌套。
IN 用来判断某个值是否在给定的结果集中，通常结合子查询来使用。当表达式与子查询返回的结果集中的某个值相等时，返回 TRUE，否则返回FALSE。 若启用了 NOT 关键字，则返回值相反。需要注意的是，子查询只能返回一列数据。
查询表中分数大于或等于80的
mysql> select name,score from info where id in (select id from info where score>=80);
+----------+-------+
| name | score |
+----------+-------+
| zhangsan | 80.00 |
| lisi | 80.00 |
| tianqi | 80.00 |
+----------+-------+
3 rows in set (0.00 sec)
查询表中分数大于或等于80的
mysql> select name,score from info where name in (select name from info where score>=80);
+----------+-------+
| name | score |
+----------+-------+
| zhangsan | 80.00 |
| lisi | 80.00 |
| tianqi | 80.00 |
+----------+-------+
3 rows in set (0.00 sec)
清空表
mysql> show tables;
+----------------+
| Tables_in_home |
+----------------+
| info |
| tmp |
| tmp1 |
+----------------+
3 rows in set (0.00 sec)
mysql> truncate table tmp;
Query OK, 0 rows affected (0.00 sec)
mysql> select * from tmp;
Empty set (0.00 sec)
子查询还可以用在 INSERT 语句中。子查询的结果集可以通过 INSERT 语句插入到其他的表中
将info表中的数据全部插入到tmp表中
mysql> insert into tmp select * from info where id in (select id from info);
Query OK, 6 rows affected (0.00 sec)
Records: 6 Duplicates: 0 Warnings: 0
mysql> select * from tmp;
+----+----------+-------+---------+
| id | name | score | address |
+----+----------+-------+---------+
| 1 | zhangsan | 80.00 | beijing |
| 2 | lisi | 80.00 | nanjing |
| 3 | wangwu | 70.00 | beijing |
| 4 | zhaoliu | 60.00 | nanjing |
| 5 | tianqi | 80.00 | beijing |
| 6 | heiba | 70.00 | nanjing |
+----+----------+-------+---------+
6 rows in set (0.00 sec)
UPDATE 语句也可以使用子查询。UPDATE 内的子查询，在 set 更新内容时，可以是单独的一列，也可以是多列。
这里使用了多表查询
mysql> update tmp set score=score -15 where id in (select id from info where score >= 60);
Query OK, 6 rows affected (0.00 sec)
Rows matched: 6 Changed: 6 Warnings: 0
mysql> select * from tmp;
+----+----------+-------+---------+
| id | name | score | address |
+----+----------+-------+---------+
| 1 | zhangsan | 65.00 | beijing |
| 2 | lisi | 65.00 | nanjing |
| 3 | wangwu | 55.00 | beijing |
| 4 | zhaoliu | 45.00 | nanjing |
| 5 | tianqi | 65.00 | beijing |
| 6 | heiba | 55.00 | nanjing |
+----+----------+-------+---------+
6 rows in set (0.00 sec)
类似 select方式的子查询，MySQL 不支持，需要多引入一层子查询
mysql> update tmp set score=score -15 where id in (select tmp.id from (select id from tmp where score >= 60)tmp);
Query OK, 3 rows affected (0.01 sec)
Rows matched: 3 Changed: 3 Warnings: 0
mysql> select * from tmp;
+----+----------+-------+---------+
| id | name | score | address |
+----+----------+-------+---------+
| 1 | zhangsan | 50.00 | beijing |
| 2 | lisi | 50.00 | nanjing |
| 3 | wangwu | 55.00 | beijing |
| 4 | zhaoliu | 45.00 | nanjing |
| 5 | tianqi | 50.00 | beijing |
| 6 | heiba | 55.00 | nanjing |
+----+----------+-------+---------+
6 rows in set (0.00 sec)
delete子查询方式，也要多一层子查询
mysql> delete from tmp where id in (select tmp.id from (select id from tmp where score=55)tmp);
Query OK, 2 rows affected (0.00 sec)
mysql> select * from tmp;
+----+----------+-------+---------+
| id | name | score | address |
+----+----------+-------+---------+
| 1 | zhangsan | 50.00 | beijing |
| 2 | lisi | 50.00 | nanjing |
| 4 | zhaoliu | 45.00 | nanjing |
| 5 | tianqi | 50.00 | beijing |
+----+----------+-------+---------+
4 rows in set (0.00 sec)
在 IN 前面还可以添加 NOT，其作用与 IN 相反，表示否 定（即不在子查询的结果集里面）。
mysql> select name,score from tmp where id not in (select id from tmp where score<50);
+----------+-------+
| name | score |
+----------+-------+
| zhangsan | 50.00 |
| lisi | 50.00 |
| tianqi | 50.00 |
+----------+-------+
3 rows in set (0.00 sec)
mysql> select name,score from tmp where id in (select id from tmp where score<50);
+---------+-------+
| name | score |
+---------+-------+
| zhaoliu | 45.00 |
+---------+-------+
1 row in set (0.00 sec)
EXIST 这个关键字在子查询时，主要用于判断子查询的结果集是否为空。如果不为空， 则返回 TRUE；反之，则返回 FALSE。
子查询语句有‘zhangsan’这个数据，所以返回值为true，执行前面的聚合函数
mysql> select count(*) as '总数' from tmp where exists (select id from tmp where name='zhangsan');
+--------+
| 总数 |
+--------+
| 4 |
+--------+
1 row in set (0.00 sec)
7 NULL 值
通常 NULL 被用来表示缺失的值，在表中该字段是没有值的。在创建表时可以使用 NOTNULL 关键字，限制某些字段不为空；不使用则默认可以为空。在向表内插入记录或者更新记录时，如果该字段没有 NOT NULL 并且没有值，这时候新记录的该字段将被保存为 NULL。
NULL 值与数字 0 或者空白（spaces）的字段是不同的， NULL 是没有值的，空白（spaces）表示一个为空的字符串类型，0表示一个整数型的数值。在 SQL 语句中，使用 IS NULL 可以判断表内的某个字段是不是 NULL 值，相反的用 IS NOT NULL 可以判断不是 NULL 值。
创建一张新表，name字段设置可以为空，验证是否设置为null值
mysql> create table num (id int(4) not null primary key auto_increment,name char(10));
Query OK, 0 rows affected (0.01 sec)
mysql> describe num;
+-------+----------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------+----------+------+-----+---------+----------------+
| id | int(4) | NO | PRI | NULL | auto_increment |
| name | char(10) | YES | | NULL | |
+-------+----------+------+-----+---------+----------------+
2 rows in set (0.00 sec)
mysql> insert into num values (1,'tom');
Query OK, 1 row affected (0.00 sec)
mysql> insert into num (id) values (2);
Query OK, 1 row affected (0.00 sec)
mysql> select * from num;
+----+------+
| id | name |
+----+------+
| 1 | tom |
| 2 | NULL |
+----+------+
2 rows in set (0.00 sec)
查询name字段为null的
mysql> select * from num where name is null;
+----+------+
| id | name |
+----+------+
| 2 | NULL |
+----+------+
1 row in set (0.00 sec)
查询name字段不为null的
mysql> select * from num where name is not null;
+----+------+
| id | name |
+----+------+
| 1 | tom |
+----+------+
1 row in set (0.00 sec)
null值和空值的区别：
- 空值的长度为 0，不占用空间的；而 NULL 值的长度是 NULL，是占用空间的。
- IS NULL 或者 IS NOT NULL，是用来判断字段是不是为 NULL 或者不是 NULL，不能查出是不是空值的。
- 空值的判断使用=’’或者<>’’来处理。
- 在通过 count()计算有多少记录数时，如果遇到 NULL 值会自动忽略掉，遇到空值会加 入到记录中进行计算