SELECT查询语句
---进行单条记录、多条记录、单表、多表、子查询……
SELECT
[ALL | DISTINCT | DISTINCTROW ]
[HIGH_PRIORITY]
[MAX_STATEMENT_TIME = N]
[STRAIGHT_JOIN]
[SQL_SMALL_RESULT] [SQL_BIG_RESULT] [SQL_BUFFER_RESULT]
[SQL_CACHE | SQL_NO_CACHE] [SQL_CALC_FOUND_ROWS]
select_expr [, select_expr ...]
[FROM table_references
[PARTITION partition_list]
[WHERE where_condition]
[GROUP BY {col_name | expr | position}
[ASC | DESC], ... [WITH ROLLUP]]
[HAVING where_condition]
[ORDER BY {col_name | expr | position}
[ASC | DESC], ...]
[LIMIT {[offset,] row_count | row_count OFFSET offset}]
[PROCEDURE procedure_name(argument_list)]
[INTO OUTFILE 'file_name'
[CHARACTER SET charset_name]
export_options
| INTO DUMPFILE 'file_name'
| INTO var_name [, var_name]]
[FOR UPDATE | LOCK IN SHARE MODE]]
select书写使用技巧：
①确认需要访问数据来自哪几张表
from来自某张表或者某几张表
join添加某张表
on表连接条件
记住一点：每关联一个表就需要加上对应的on条件（on条件就是主外键条件）
②通过where条件过滤数据
③确认需求里面是否有分组聚合的含义
分组：group by
聚合：聚合函数
聚合条件过滤：having
④是否需要排序
order by
1、查询某张表所有数据
mysql> select * from temp;
解析：*代表所有列，temp代表表名，不带条件就查询所有数据
2、查询指定列和条件的数据
mysql> select name,age from temp where age = 22;
解析：查询name和age这两列，age 等于22的数据。
3、对查询的数据进行运算操作
mysql> select age+2,age/2,age-2,age*2 from temp where age-2 > 22;
mysql> select PLAYRNO,AMOUNT,AMOUNT*6.5 “Ren Min Bi” from PENALTIES;
解析：查询AMOUNT列数据乘6.5并改列名为Ren Min Bi
使用小括号可以改变运算的优先级
4、concat函数，字符串连接
mysql> select NAME,concat(TOWN,STREET,HOUSENO) “player Home Address” from PLAYERS;
解析：利用concat函数将TOWN,STREET,HOUSENO三列的字符串连接起来，别名列为player Home Address(可以在concat里加' '进行分隔)
注意：concat和null进行连接，会导致连接后的数据成为null
mysql> select * from t1;
+------+--------+--------+
| id | First | Last |
+------+--------+--------+
| 1 | zhang | jiacai |
| 2 | linghu | NULL |
+------+--------+--------+
mysql> select id,concat(First,' ',Last) from t1;
+------+------------------------+
| id | concat(First,' ',Last) |
+------+------------------------+
| 1 | zhang jiacai |
| 2 | NULL |
+------+------------------------+
concat_ws函数，指定分隔符的字符串连接
mysql> select id,concat_ws(':',First,Last) "Full Name" from t1;
+------+--------------+
| id | Full Name |
+------+--------------+
| 1 | zhang:jiacai |
| 2 | linghu |
+------+--------------+
圆括号里的第一个位置用来指定字符串连接的分隔符
5、as 列别名(或省略，留空格)
mysql> select id as 'num' from t1;
mysql> select id 'num',First from t1;
6、distinct关键字去掉重复数据
mysql> select * from t2;
+------+
| num |
+------+
| 1 |
| 1 |
| 1 |
| 2 |
+------+
mysql> select distinct num from t2;
+------+
| num |
+------+
| 1 |
| 2 |
+------+
distinct 多列：去重的是同行多列组合的重复数据
mysql> select distinct id, age from temp;
7、where条件查询
where语句中的条件比较：大于>、大于等于>=、小于<、小于等于<=、等于=、不等于<>
mysql> select * from tableName where a>2 or a>=3 or a<5 or a<=6 or a=7 or a<>0;
8、and 并且、or 或者、not非
mysql> select * from temp where age>20 and name=‘jack’;
解析：查询name等于jack并且年龄大于20的
mysql> select * from tmep where name=‘jack’ or name=‘jackson’;
解析：查询name是jack或是jackson的
mysql> select * from temp where not (age > 20);
解析：取小于等于20的数据
mysql> select * from temp where id not in(1, 2);
解析：查询id数不是1，也不是2的
9、between v1 and v2：v1和v2之间
mysql> select * form temp where age between 20 and 25;
解析：查询age在20和25之间的
10、in 查询：多个条件 类似于or
mysql> select * from temp where id in (1, 2, 3);
解析：查询id在括号中出现的数据
11、like 模糊查询
%:替代0个或多个字符
_：替代一个字符
mysql> select * from temp where name like ‘j%’;
解析：查询name以j开头的(%通配所有)
mysql> select * from temp where name like ‘%k%’;
解析：查询name包含k的
escape转义
mysql> select * from temp where name like ‘\_%’ escape ‘’;
解析：指定为转义字符，上面的就可以查询name中以“_”开头的数据
12、is null、is not null
mysql> select * from temp where name is null;
解析：查询为null的数据
mysql> select * from temp where name is not null;
解析：查询不为null的数据
13、order by排序：desc降序、asc升序
mysql> select * from temp order by id; (默认asc升排序)
mysql> select * from temp order by id desc; (指定降序排)
多列组合
mysql> select * from temp order by id, age;
14、limit子句：从结果集中选取最前面或最后面的几行
通常和order by连用，放其后面
limit <获取的行数> [OFFSET <跳过的行数>]
limit [<跳过的行数>,] <获取的行数>
mysql> select playerno,name from players order by playerno asc limit 3,5;
mysql> select playerno,name from players order by playerno asc limit 5 offset 3;
解析：跳过前面的3行，从第4行开始取，取5行
注意：MySQL5.7 doesn't yet support 'LIMIT & IN/ALL/ANY/SOME subquery'
15、group by、having 分组聚合
select [聚合函数] 字段名 from 表名
[where 查询条件]
[group by 字段名]
[having 过滤条件]
mysql> select salary,count(*) from salary_tab
-> where salary>=2000
-> group by salary
-> having count(*)>=0;
+---------+----------+
| salary | count(*) |
+---------+----------+
| 2000.00 | 1 |
| 3000.00 | 1 |
+---------+----------+
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