一. 子查询
子查询就是指在一个select语句中嵌套另一个select语句。同时，子查询必须包含括号。
MySQL 5.6.x版本之前，MySQL的子查询性能较差，但是从5.6开始，不存在性能差的问题。
select a from t1 where a > any(select a from t2);
select a from t1是外部查询(outer query)(select a from t2)是子查询(subquery)
一般说来，子查询嵌套于外部查询中，可以将两个或两个以上的子查询进行嵌套
1. 子查询的使用
1.1. ANY / SOME
如果外部查询的列的结果和子查询的列的结果比较得到为True的话，则返回比较值为True的外查询的记录
(gcdb@localhost) 17:37:05 [mytest]> select * from t1;
+------+
| a |
+------+
| 1 |
| 3 |
| 4 |
| 5 |
| 7 |
+------+
5 rows in set (0.00 sec)
(gcdb@localhost) 17:37:22 [mytest]> select * from t2;
+------+
| a |
+------+
| 2 |
| 4 |
| 8 |
| 10 |
+------+
4 rows in set (0.00 sec)
(gcdb@localhost) 17:39:33 [mytest]> select a from t1 where a > any(select a from t2);
+------+
| a |
+------+
| 3 |
| 4 |
| 5 |
| 7 |
+------+
4 rows in set (0.00 sec)
-- 这个查询可以解释为，t1表内a列的值大于t2表中a列的`任意(any)`一个值`t1.a > any(t2.a) == true`,则返回t1.a的记录
ANY关键词必须与一个比较操作符一起使用：=,>,<,>=,<=,<>(这个是!=的意思)
子查询中
SOME和ANY是同一个意思
1.2. IN
in是ANY的一种特殊情况："in" equals "= any"
(gcdb@localhost) 17:58:21 [mytest]> select a from t1 where a = any(select a from t2); -- t1.a==t2.a 的只有4
+------+
| a |
+------+
| 4 |
+------+
1 row in set (0.00 sec)
(gcdb@localhost) 17:57:01 [mytest]> select a from t1 where a in (select a from t2); -- in的结果等同于 =any 的结果
+------+
| a |
+------+
| 4 |
+------+
1 row in set (0.00 sec)
select a from s1 where a in (select a in t2);是用的比较多的一种语法
1.3. ALL
如果外部查询的列的结果和子查询的列的所有结果比较得到为True的话，则返回比较值为True的（外查询）的记录
(gcdb@localhost) 17:55:03 [mytest]> select a from t2 where a >all(select a from t1);
+------+
| a |
+------+
| 8 |
| 10 |
+------+
2 rows in set (0.00 sec)
ALL关键词必须与一个比较操作符一起使用
NOT IN是<> ALL的别名
2. 子查询的分类
-
独立子查询
- 不依赖外部查询而运行的子查询
(gcdb@localhost) 18:00:04 [mytest]> select a from t1 where a in (1,2,3,4,5); +------+ | a | +------+ | 1 | | 3 | | 4 | | 5 | +------+ 4 rows in set (0.00 sec) -
相关子查询
- 引用了外部查询列的子查询
-- 在这个例子中，子查询中使用到了外部的列t2.a (gcdb@localhost) 18:00:48 [mytest]> select a from t1 where a in (select * from t2 where t1.a = t2.a); +------+ | a | +------+ | 4 | +------+ 1 row in set (0.00 sec)
3. 子查询的优化
-
MySQL5.6之前
- 在
MySQL5.6之前，优化器会把子查询重写成exists的形式
select a from t1 where a in (select a from t2); -- 这个是一条独立的子查询，时间复杂度 O(M+N) -- -- 经过优化器重写后 -- select a from t1 where exists (select 1 from t2 where t1.a = t2.a); -- 这是相关子查询，复杂度O(M*N + M)- 在
MySQL 5.6之前，部分的子查询需要重写成join的形式 （注意表的大小）
mysql> select t1.a from t1 join t2 on t1.a = t2.a; +------+ | a | +------+ | 4 | +------+ 1 row in set (0.00 sec) - 在
-
MySQL 5.6之后
在MySQL 5.6之后，优化器不会将子查询重写成exists的形式，而是自动优化，性能有了大幅提升
4. 包含NULL值的NOT IN
mysql> select null in ('a', 'b', null);
+--------------------------+
| null in ('a', 'b', null) |
+--------------------------+
| NULL |
+--------------------------+
1 row in set (0.00 sec)
MySQL数据库的
比较操作，除了返回1(True),0(False)之外，还会返回NULL
NULL和NULL的比较，返回的还是NULL
mysql> select null not in ('a', 'b', null);
+------------------------------+
| null not in ('a', 'b', null) |
+------------------------------+
| NULL | -- null不在('a', 'b', null)中，返回的还是null，因为有null和null的比较
+------------------------------+
1 row in set (0.00 sec)
mysql> select 'a' not in ('a', 'b', null);
+-----------------------------+
| 'a' not in ('a', 'b', null) |
+-----------------------------+
| 0 | -- a 不在 ('a', 'b', null)中，返回0,即False
+-----------------------------+
1 row in set (0.00 sec)
mysql> select 'c' not in ('a', 'b');
+-----------------------+
| 'c' not in ('a', 'b') |
+-----------------------+
| 1 | -- 这个返回值可以理解 'c'不在('a', 'b')中，返回1，即为True
+-----------------------+
1 row in set (0.00 sec)
mysql> select 'c' not in ('a', 'b', null);
+-----------------------------+
| 'c' not in ('a', 'b', null) |
+-----------------------------+
| NULL | -- 理论上应该是返回1，即True的。但是包含了null值。则返回null
+-----------------------------+
1 row in set (0.00 sec)
对于包含了
NULL值的IN操作，总是返回True或者NULL
NOT IN返回NOT True (False)或者NOT NULL (NULL)
--
-- SQL语句一 使用 EXISTS
--
select customerid, companyname
from customers as A
where country = 'Spain'
and not exists
( select * from orders as B
where A.customerid = B.customerid );
--
-- SQL语句二 使用 IN
--
select customerid, companyname
from customers as A
where country = 'Spain'
and customerid not in (select customerid from orders);
-----
-- 当结果集合中没有NULL值时，上述两条SQL语句查询的结果是一致的
-----
--
-- 插入一个NULL值
--
insert into orders(orderid) values (null);
-----
-- SQL语句1 : 返回和之前一致
-- SQL语句2 : 返回为空表，因为子查询返回的结果集中存在NULL值。not in null 永远返回False或者NULL
-- 此时 where (country = 'Spain' and (False or NULL)) 为 False OR NULL，条件永远不匹配
-----
--
-- SQL语句2 改写后
--
select customerid, companyname
from customers as A
where country = 'Spain'
and customerid not in (select customerid from orders
where customerid is not null); -- 增加这个过滤条件，使用is not，而不是<>
--
-- 和 null比较，使用is和is not， 而不是 = 和 <>
--
mysql> select null = null;
+-------------+
| null = null |
+-------------+
| NULL |
+-------------+
1 row in set (0.00 sec)
mysql> select null <> null;
+--------------+
| null <> null |
+--------------+
| NULL |
+--------------+
1 row in set (0.00 sec)
mysql> select null is null;
+--------------+
| null is null |
+--------------+
| 1 | -- 返回 True
+--------------+
1 row in set (0.00 sec)
mysql> select null is not null;
+-------------------+
| null is not null |
+-------------------+
| 0 | -- 返回 False
+-------------------+
1 row in set (0.00 sec)
EXISTS不管返回值是什么，而是看是否有行返回，所以EXISTS中子查询都是select *、select 1等，因为只关心返回是否有行（结果集）
二. INSERT
(gcdb@localhost) 10:46:17 [mytest]> select * from t1;
+------+
| a |
+------+
| 1 |
| 3 |
| 4 |
| 5 |
| 7 |
+------+
5 rows in set (0.00 sec)
(gcdb@localhost) 10:46:36 [mytest]> insert into t1 values(9); -- 插入一个值
Query OK, 1 row affected (0.01 sec)
(gcdb@localhost) 10:46:44 [mytest]> insert into t1(a) values(9),(11); -- 插入多个值，MySQL独有
Query OK, 2 rows affected (0.00 sec)
Records: 2 Duplicates: 0 Warnings: 0
(gcdb@localhost) 10:47:11 [mytest]> insert into t1(a) select 13; -- insert XXX select XXX 语法，MySQ独有
Query OK, 1 row affected (0.00 sec)
Records: 1 Duplicates: 0 Warnings: 0
(gcdb@localhost) 10:53:34 [mytest]> select * from t1;
+------+
| a |
+------+
| 1 |
| 3 |
| 4 |
| 5 |
| 7 |
| 9 |
| 11 |
| 13 |
+------+
8 rows in set (0.00 sec)
(gcdb@localhost) 10:46:52 [mytest]> create table t3(a int, b int); -- 有多个列
Query OK, 0 rows affected (0.15 sec)
(gcdb@localhost) 10:47:24 [mytest]> insert into t3 select 12; -- 没有指定列，报错
ERROR 1136 (21S01): Column count doesn't match value count at row 1
(gcdb@localhost) 10:47:48 [mytest]> insert into t3(a) select 12; -- 指定列a
Query OK, 1 row affected (0.00 sec)
Records: 1 Duplicates: 0 Warnings: 0
(gcdb@localhost) 10:48:08 [mytest]> insert into t3(a,b) select 11,12; -- 不指定列，但是插入值匹配列的个数和类型
Query OK, 1 row affected (0.00 sec)
Records: 1 Duplicates: 0 Warnings: 0
(gcdb@localhost) 10:48:21 [mytest]> select * from t3;
+------+------+
| a | b |
+------+------+
| 12 | NULL |
| 11 | 12 |
+------+------+
2 rows in set (0.00 sec)
(gcdb@localhost) 10:49:19 [mytest]> insert into t3 select * from t3; -- 从t3表中查询数据并插回t3中
Query OK, 2 rows affected (0.00 sec)
Records: 2 Duplicates: 0 Warnings: 0
(gcdb@localhost) 10:55:49 [mytest]> insert into t3(a) select a from t2; -- 从t2表中查询数据并插入到t3(a)中，注意指定列
Query OK, 4 rows affected (0.00 sec)
Records: 4 Duplicates: 0 Warnings: 0
(gcdb@localhost) 10:59:02 [mytest]> select * from t3;
+------+------+
| a | b |
+------+------+
| 12 | NULL |
| 11 | 12 |
| 12 | NULL |
| 11 | 12 |
| 2 | NULL |
| 4 | NULL |
| 8 | NULL |
| 10 | NULL |
+------+------+
8 rows in set (0.00 sec)
--
-- 如果想快速增长表格中的数据，可以使用如下方法，使得数据成倍增长
--
mysql> insert into t3 select * from t3;
Query OK, 5 rows affected (0.03 sec) -- 插入了5列
Records: 5 Duplicates: 0 Warnings: 0
mysql> select * from t3;
+------+------+
| a | b |
+------+------+
| 8 | NULL |
| 8 | 9 |
| NULL | 5 |
| NULL | 4 |
| NULL | 3 |
| 8 | NULL |
| 8 | 9 |
| NULL | 5 |
| NULL | 4 |
| NULL | 3 |
+------+------+
10 rows in set (0.00 sec)
三. DELETE
(gcdb@localhost) 11:02:10 [mytest]> delete from t3 where b is null; -- 根据过滤条件删除
Query OK, 6 rows affected (0.00 sec)
(gcdb@localhost) 11:02:16 [mytest]> select * from t3;
+------+------+
| a | b |
+------+------+
| 11 | 12 |
| 11 | 12 |
+------+------+
2 rows in set (0.00 sec)
(gcdb@localhost) 11:02:48 [mytest]> delete from t3; -- 删除整个表
Query OK, 2 rows affected (0.00 sec)
(gcdb@localhost) 11:03:17 [mytest]> select * from t3;
Empty set (0.00 sec)
四. UPDATE
(gcdb@localhost) 11:03:19 [mytest]> insert into t3 select 1,2;
Query OK, 1 row affected (0.00 sec)
Records: 1 Duplicates: 0 Warnings: 0
(gcdb@localhost) 11:03:54 [mytest]> insert into t3 select 2,3;
Query OK, 1 row affected (0.00 sec)
Records: 1 Duplicates: 0 Warnings: 0
(gcdb@localhost) 11:03:58 [mytest]> insert into t3 select 3,4;
Query OK, 1 row affected (0.00 sec)
Records: 1 Duplicates: 0 Warnings: 0
(gcdb@localhost) 11:04:02 [mytest]> select * from t3;
+------+------+
| a | b |
+------+------+
| 1 | 2 |
| 2 | 3 |
| 3 | 4 |
+------+------+
3 rows in set (0.00 sec)
(gcdb@localhost) 11:04:30 [mytest]> update t3 set a =10 where a=1;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0
(gcdb@localhost) 11:05:02 [mytest]> select * from t3;
+------+------+
| a | b |
+------+------+
| 10 | 2 |
| 2 | 3 |
| 3 | 4 |
+------+------+
3 rows in set (0.00 sec)
--
-- 关联后更新
--
(gcdb@localhost) 11:05:07 [mytest]> select * from t2;
+------+
| a |
+------+
| 2 | --和t3中的a列2相等
| 4 |
| 8 |
| 10 | --和t3中的a列10相等
+------+
4 rows in set (0.00 sec)
(gcdb@localhost) 11:06:05 [mytest]> select * from t3;
+------+------+
| a | b |
+------+------+
| 10 | 2 | -- 和t2中的10相等
| 2 | 3 | -- 和t2中的2相等
| 3 | 4 |
+------+------+
3 rows in set (0.00 sec)
(gcdb@localhost) 11:08:08 [mytest]> update t2 join t3 on t2.a = t3.a set t2.a=1000; -- 先得到t2.a=t3.a的结果然后将结果集中的t2.a设置为100
Query OK, 2 rows affected (0.00 sec)
Rows matched: 2 Changed: 2 Warnings: 0
(gcdb@localhost) 11:08:16 [mytest]> select * from t2;
+------+
| a |
+------+
| 1000 | -- 该行原先2被更新成100
| 4 |
| 8 |
| 1000 | -- 该行原先10被更新成100
+------+
4 rows in set (0.01 sec)
五. REPLACE
(gcdb@localhost) 11:18:41 [mytest]> create table t4(a int primary key auto_increment,b int);
Query OK, 0 rows affected (0.01 sec)
(gcdb@localhost) 11:18:47 [mytest]> insert into t4 values(null,1);
Query OK, 1 row affected (0.00 sec)
(gcdb@localhost) 11:19:17 [mytest]> insert into t4 values(null,2);
Query OK, 1 row affected (0.00 sec)
(gcdb@localhost) 11:19:19 [mytest]> insert into t4 values(null,3);
Query OK, 1 row affected (0.00 sec)
(gcdb@localhost) 11:19:22 [mytest]> select * from t4;
+---+------+
| a | b |
+---+------+
| 1 | 1 |
| 2 | 2 |
| 3 | 3 |
+---+------+
3 rows in set (0.00 sec)
(gcdb@localhost) 11:20:01 [mytest]> insert into t4 values(1,10); -- error，主键值1，重复
ERROR 1062 (23000): Duplicate entry '1' for key 'PRIMARY'
(gcdb@localhost) 11:20:28 [mytest]> replace into t4 values(1,10); -- 替换该主键对应的值
Query OK, 2 rows affected (0.00 sec) -- 两行记录受到影响
(gcdb@localhost) 11:20:54 [mytest]> select * from t4;
+---+------+
| a | b |
+---+------+
| 1 | 10 | -- 已经被更新
| 2 | 2 |
| 3 | 3 |
+---+------+
3 rows in set (0.00 sec)
-----
-- replace的原理是：先delete，在insert ；注意：需要delete和insert权限
-----
(gcdb@localhost) 11:24:23 [mytest]> replace into t4 values(5,15); -- 没有替换对象时，类似插入效果
Query OK, 1 row affected (0.03 sec) -- 只影响1行
(gcdb@localhost) 11:24:32 [mytest]> select * from t4;
+---+------+
| a | b |
+---+------+
| 1 | 10 |
| 2 | 2 |
| 3 | 3 |
| 5 | 15 | -- 新插入一行
+---+------+
4 rows in set (0.00 sec)
--
-- replace原理更明显的例子
--
(gcdb@localhost) 11:24:35 [mytest]> create table t6
-> (a int primary key,
-> b int auto_increment, -- b是auto_increment的int型数据
-> c int,key(b));
Query OK, 0 rows affected (0.01 sec)
(gcdb@localhost) 11:26:54 [mytest]> insert into t6 values(1,null,10),(2,null,20),(3,null,30);
Query OK, 3 rows affected (0.00 sec)
Records: 3 Duplicates: 0 Warnings: 0
(gcdb@localhost) 11:28:08 [mytest]> select * from t6;
+---+---+------+
| a | b | c | --b类为自增列
+---+---+------+
| 1 | 1 | 10 |
| 2 | 2 | 20 |
| 3 | 3 | 30 |
+---+---+------+
3 rows in set (0.00 sec)
(gcdb@localhost) 11:29:14 [mytest]> replace into t6 values(1,null,99);
Query OK, 2 rows affected (0.00 sec)
(gcdb@localhost) 11:29:40 [mytest]> select * from t6;
+---+---+------+
| a | b | c |
+---+---+------+
| 1 | 4 | 99 | --当a=1时,c列值10被替换为99和b列值3自增加1为4
| 2 | 2 | 20 |
| 3 | 3 | 30 |
+---+---+------+
3 rows in set (0.00 sec)
-----
--
-- insert on duplicate 效果和 replace类似
--
(gcdb@localhost) 11:29:50 [mytest]> select * from t4;
+---+------+
| a | b |
+---+------+
| 1 | 10 |
| 2 | 2 |
| 3 | 3 |
| 5 | 15 |
+---+------+
4 rows in set (0.00 sec)
(gcdb@localhost) 11:33:31 [mytest]> insert into t4 values(1,1); -- 插入报错，存在key为1的记录
ERROR 1062 (23000): Duplicate entry '1' for key 'PRIMARY'
(gcdb@localhost) 11:34:26 [mytest]> insert into t4 values(1,1) on duplicate key update b=1; -- 带上on duplicate参数（非SQL标准，不推荐）
Query OK, 2 rows affected (0.00 sec)
(gcdb@localhost) 11:34:31 [mytest]> select * from t4;
+---+------+
| a | b |
+---+------+
| 1 | 1 | - 该行的b列从10被替换成1
| 2 | 2 |
| 3 | 3 |
| 5 | 15 |
+---+------+
4 rows in set (0.00 sec)
--
-- insert ignore
--
(gcdb@localhost) 11:34:34 [mytest]> insert ignore into t4 values(1,1); -- 忽略重复的错误
Query OK, 0 rows affected, 1 warning (0.00 sec)
(gcdb@localhost) 11:36:37 [mytest]> show warnings;
+---------+------+---------------------------------------+
| Level | Code | Message |
+---------+------+---------------------------------------+
| Warning | 1062 | Duplicate entry '1' for key 'PRIMARY' |
+---------+------+---------------------------------------+
1 row in set (0.00 sec)
六. UNION
- UNION的作用是
将两个查询的结果集进行合并。 - UNION必须由
两条或两条以上的SELECT语句组成，语句之间用关键字UNION分隔。 - UNION中的每个查询必须包含相同的列（
类型相同或可以隐式转换）、表达式或聚集函数。
(gcdb@localhost) 11:14:02 [mytest]> create table t_union01(a int,b int);
Query OK, 0 rows affected (0.00 sec)
(gcdb@localhost) 11:14:31 [mytest]> create table t_union02(a int,b int);
Query OK, 0 rows affected (0.01 sec)
(gcdb@localhost) 11:15:41 [mytest]> insert into t_union01 values(1,2),(2,3),(3,4),(99,100);
Query OK, 4 rows affected (0.01 sec)
Records: 4 Duplicates: 0 Warnings: 0
(gcdb@localhost) 11:16:04 [mytest]> insert into t_union02 values(10,20),(20,30),(30,40),(99,100);
Query OK, 4 rows affected (0.00 sec)
Records: 4 Duplicates: 0 Warnings: 0
(gcdb@localhost) 11:16:26 [mytest]> select * from t_union01;
+------+------+
| a | b |
+------+------+
| 1 | 2 |
| 2 | 3 |
| 3 | 4 |
| 99 | 100 | --t_union01 中的99, 100
+------+------+
4 rows in set (0.00 sec)
(gcdb@localhost) 11:16:42 [mytest]> select * from t_union02;
+------+------+
| a | b |
+------+------+
| 10 | 20 |
| 20 | 30 |
| 30 | 40 |
| 99 | 100 | --t_union02 中的99, 100
+------+------+
4 rows in set (0.00 sec)
(gcdb@localhost) 11:22:16 [mytest]> select * from t_union02
-> union
-> select * from t_union01;
+------+------+
| a | b |
+------+------+
| 10 | 20 |
| 20 | 30 |
| 30 | 40 |
| 99 | 100 | -- 只出现了一次 99, 100，union会去重
| 1 | 2 |
| 2 | 3 |
| 3 | 4 |
+------+------+
7 rows in set (0.00 sec)
(gcdb@localhost) 11:22:35 [mytest]> select * from t_union02 union all select * from t_union01; -- 使用 union all 显示不去重
+------+------+
| a | b |
+------+------+
| 10 | 20 |
| 20 | 30 |
| 30 | 40 |
| 99 | 100 |
| 1 | 2 |
| 2 | 3 |
| 3 | 4 |
| 99 | 100 |
+------+------+
8 rows in set (0.00 sec)
如果知道数据本身具有唯一性，没有重复，则建议使用
union all，因为union会做去重操作，性能会比union all要低
七. 关联更新和行号查询
7.1. 关联更新
(gcdb@localhost) 11:38:36 [mytest]> create table t5 (a int, b int);
Query OK, 0 rows affected (0.14 sec)
(gcdb@localhost) 11:39:38 [mytest]> insert into t5 values(1,1);
Query OK, 1 row affected (0.03 sec)
(gcdb@localhost) 11:39:49 [mytest]> select * from t5;
+------+------+
| a | b |
+------+------+
| 1 | 1 |
+------+------+
1 row in set (0.00 sec)
(gcdb@localhost) 11:39:51 [mytest]> update t5 set a= a+1,b=a where a=1;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0
(gcdb@localhost) 11:40:03 [mytest]> select * from t5;
+------+------+
| a | b |
+------+------+
| 2 | 2 | -- SQL Server和Oracle中得到的值是2, 1
+------+------+
1 row in set (0.00 se
7.2. 显示行号(RowNumber)
--
-- 方法一
--
(gcdb@localhost) 11:41:08 [mytest]> use employees;
Database changed
(gcdb@localhost) 11:42:15 [employees]> set @rn:=0; -- 产生 SESSION(会话)级别的变量
Query OK, 0 rows affected (0.00 sec)
(gcdb@localhost) 11:42:21 [employees]> select @rn:=@rn+1 as rownumber, emp_no, gender from employees limit 10; -- @rn:=1 是赋值的意思
+-----------+--------+--------+
| rownumber | emp_no | gender |
+-----------+--------+--------+
| 1 | 10001 | M |
| 2 | 10002 | F |
| 3 | 10003 | M |
| 4 | 10004 | M |
| 5 | 10005 | M |
| 6 | 10006 | F |
| 7 | 10007 | F |
| 8 | 10008 | M |
| 9 | 10009 | F |
| 10 | 10010 | F |
+-----------+--------+--------+
10 rows in set (0.00 sec)
--
-- 方法二 （推荐）
--
(gcdb@localhost) 11:42:31 [employees]> select @rn1:=@rn1+1 as rownumber, emp_no, gender from employees, (select @rn1:=0) as a limit 10;
+-----------+--------+--------+
| rownumber | emp_no | gender |
+-----------+--------+--------+
| 1 | 10001 | M |
| 2 | 10002 | F |
| 3 | 10003 | M |
| 4 | 10004 | M |
| 5 | 10005 | M |
| 6 | 10006 | F |
| 7 | 10007 | F |
| 8 | 10008 | M |
| 9 | 10009 | F |
| 10 | 10010 | F |
+-----------+--------+--------+
10 rows in set (0.00 sec)
-- MySQL 自定义变量，根据每一记录进行变化的
(gcdb@localhost) 11:44:55 [employees]> select @rn1:=0;
+---------+
| @rn1:=0 |
+---------+
| 0 | -- 只有一行记录
+---------+
1 row in set (0.00 sec)
-- 相当于 把 employees 和 (select @rn1:=0)做了笛卡尔积，然后使用@rn1:=@rn + 1，根据每行进行累加
--
-- ":=" 和 "="
--
(gcdb@localhost) 11:42:15 [employees]> set @rn:=0; -- 赋值为0
Query OK, 0 rows affected (0.00 sec)
(gcdb@localhost) 11:44:55 [employees]> select @rn1:=0;
+---------+
| @rn1:=0 |
+---------+
| 0 |
+---------+
1 row in set (0.00 sec)
(gcdb@localhost) 11:46:37 [employees]> set @a:=100; -- 赋值为100
Query OK, 0 rows affected (0.00 sec)
(gcdb@localhost) 11:46:54 [employees]> select @a;
+------+
| @a |
+------+
| 100 |
+------+
1 row in set (0.00 sec)
(gcdb@localhost) 11:49:25 [employees]> select @a=99; -- 进行比较
+-------+
| @a=99 |
+-------+
| 0 |
+-------+
1 row in set (0.00 sec)
- 使用子查询实现RowNumber
-
思路
- 假设当前在第N行记录，通过主键emp_no遍历有多少行的记录
小于等于当前行,即为当前行的行数
- 假设当前在第N行记录，通过主键emp_no遍历有多少行的记录
-
SQL语句
(gcdb@localhost) 12:07:39 [employees]> SELECT (SELECT COUNT(1) FROM employees b WHERE b.emp_no <= a.emp_no ) AS row_number, emp_no,CONCAT(last_name," ",first_name) name,gender,hire_date FROM employees a ORDER BY emp_no LIMIT 10;
+------------+--------+--------------------+--------+------------+
| row_number | emp_no | name | gender | hire_date |
+------------+--------+--------------------+--------+------------+
| 1 | 10001 | Facello Georgi | M | 1986-06-26 |
| 2 | 10002 | Simmel Bezalel | F | 1985-11-21 |
| 3 | 10003 | Bamford Parto | M | 1986-08-28 |
| 4 | 10004 | Koblick Chirstian | M | 1986-12-01 |
| 5 | 10005 | Maliniak Kyoichi | M | 1989-09-12 |
| 6 | 10006 | Preusig Anneke | F | 1989-06-02 |
| 7 | 10007 | Zielinski Tzvetan | F | 1989-02-10 |
| 8 | 10008 | Kalloufi Saniya | M | 1994-09-15 |
| 9 | 10009 | Peac Sumant | F | 1985-02-18 |
| 10 | 10010 | Piveteau Duangkaew | F | 1989-08-24 |
+------------+--------+--------------------+--------+------------+
10 rows in set (0.59 sec)
-- 假设当前在第5行
(gcdb@localhost) 12:08:38 [employees]> select b.emp_no from employees.employees as b order by b.emp_no limit 5;
+--------+
| emp_no |
+--------+
| 10001 |
| 10002 |
| 10003 |
| 10004 |
| 10005 | -- 第5行的emp_no是10005
+--------+
5 rows in set (0.00 sec)
(gcdb@localhost) 12:10:28 [employees]> select count(*) from employees.employees as b where b.emp_no<= 10005 order by b.emp_no;
--查找小于等于5的行数有几行
+----------+
| count(*) |
+----------+
| 5 | -- 小于等于10005的记录有5行，则5就是10005该行记录的行号
+----------+
1 row in set (0.00 sec)
-- 将该子查询的结果即可作为RowNumber,子查询循环多次，不推荐使用。
-- 推荐使用下面这种方法
(gcdb@localhost) 12:12:01 [employees]> SELECT @a:=@a+1 AS row_number,emp_no,CONCAT(last_name," ",first_name) name,gender,hire_date FROM employees,(SELECT @a:=0) AS a LIMIT 10;
+------------+--------+--------------------+--------+------------+
| row_number | emp_no | name | gender | hire_date |
+------------+--------+--------------------+--------+------------+
| 1 | 10001 | Facello Georgi | M | 1986-06-26 |
| 2 | 10002 | Simmel Bezalel | F | 1985-11-21 |
| 3 | 10003 | Bamford Parto | M | 1986-08-28 |
| 4 | 10004 | Koblick Chirstian | M | 1986-12-01 |
| 5 | 10005 | Maliniak Kyoichi | M | 1989-09-12 |
| 6 | 10006 | Preusig Anneke | F | 1989-06-02 |
| 7 | 10007 | Zielinski Tzvetan | F | 1989-02-10 |
| 8 | 10008 | Kalloufi Saniya | M | 1994-09-15 |
| 9 | 10009 | Peac Sumant | F | 1985-02-18 |
| 10 | 10010 | Piveteau Duangkaew | F | 1989-08-24 |
+------------+--------+--------------------+--------+------------+
10 rows in set (0.00 sec)
3.查询employees表下基层用户的最近详细信息（员工号,员工名字，职位，部门，工资）
关于
Group By在《SQL必知必会》中提及的部分规定:
GROUP BY子句中列出的每一列都必须是检索列或有效的表达式（但不能是聚集函数),如果在SELECT中使用表达式，则必须在GROUP BY子句中指定相同的表达式不能使用别名。
除聚集计算语句外，SELECT语句中的每一列都必须在GROUP BY子句中给出。
SELECT
e.emp_no AS 员工号,
CONCAT(last_name, ' ', first_name) AS 姓名,
t.title AS 职位,
dp.dept_name AS 部门,
s.salary AS 工资
FROM
employees e
LEFT JOIN
dept_manager d ON e.emp_no = d.emp_no
LEFT JOIN
(SELECT
emp_no, title, from_date, to_date
FROM
titles
WHERE
(emp_no , from_date, to_date) IN (SELECT
emp_no, MAX(from_date), MAX(to_date)
FROM
titles AS b
GROUP BY b.emp_no)) t ON t.emp_no = e.emp_no
LEFT JOIN
(SELECT
dept_no, emp_no, from_date, to_date
FROM
dept_emp
WHERE
(emp_no , from_date, to_date) IN (SELECT
emp_no, MAX(from_date), MAX(to_date)
FROM
dept_emp AS b
GROUP BY b.emp_no)) de ON de.emp_no = e.emp_no
LEFT JOIN
(SELECT
emp_no, salary, from_date, to_date
FROM
salaries
WHERE
(emp_no , from_date, to_date) IN (SELECT
emp_no, MAX(from_date), MAX(to_date)
FROM
salaries AS b
GROUP BY b.emp_no)) s ON s.emp_no = e.emp_no
LEFT JOIN
departments dp ON dp.dept_no = de.dept_no
WHERE
d.emp_no IS NULL
LIMIT 10;
--
-- 改进的子查询语句 - 1
--
SELECT
emp_no, title, from_date, to_date
FROM
titles
WHERE
(emp_no , from_date, to_date) IN
(
SELECT
emp_no, MAX(from_date), MAX(to_date) -- 因为数据本身的问题，这里from_date和to_date都要
FROM
titles AS b
GROUP BY b.emp_no
) -- 这个子查询表示以emp_no分类，找到最大（最近）的from_date和to_date
-- 而where条件在这个最大的基础上，过滤出我们要的title。(salary同理)
--
-- 改进的子查询语句 - 2
--
SELECT
emp_no, title, from_date, to_date
FROM
titles AS a
WHERE
(from_date, to_date) = (SELECT
MAX(from_date), MAX(to_date) -- 同样使用from_date和to_date
FROM
titles AS b
WHERE
a.emp_no = b.emp_no -- 这个是一个关联子查询
GROUP BY b.emp_no);