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nullmysql> 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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