012:子查詢和增刪改查
阿新 • • 發佈:2017-12-08
err eat 多個 cal mat 行數 分類 查詢 同時
一. 子查詢
子查詢就是指在一個select語句中嵌套另一個select語句。同時,子查詢必須包含括號。
MySQL 5.6.x
版本之前,MySQL的子查詢性能較差,但是從5.6開始,不存在性能差的問題。
select a from t1 where a > any(select a from t2);
select a from t1
是外部查詢(outer query)(select a from t2)
是子查詢(subquery)
一般說來,子查詢嵌套於外部查詢中,可以將兩個或兩個以上的子查詢進行嵌套
1. 子查詢的使用
1.1. ANY / SOME
如果外部查詢的列的結果和子查詢的列的結果比較得到為True的話,則返回比較值為True的(外查詢)的記錄
(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)
-- 這個查詢可以解釋為,t1表內a列的值大於t2表中a列的任意(any)一個值(t1.a > any(t2.a) == true),則返回t1.a的記錄
ANY
關鍵詞必須與一個比較操作符
一起使用:=
,>
,<
,>=
,<=
,<>
(這個是!=的意思)
子查詢中
SOME
和ANY
是同一個意思
1.2. IN
in
是ANY
的一種特殊情況:"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的結果等同於 =any 的結果
+------+
| a |
+------+
| 4 |
+------+
1 row in set (0.00 sec)
select a from s1 where a in (select a in t2);
是用的比較多的一種語法
1.3. ALL
如果外部查詢的列的結果和子查詢的列的所有結果
比較得到為True的話,則返回比較值為True的(外查詢)的記錄
(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
關鍵詞必須與一個比較操作符
一起使用
NOT IN
是<> ALL
的別名
2. 子查詢的分類
- 獨立子查詢
- 不依賴外部查詢而運行的子查詢
(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)
- 相關子查詢
- 引用了外部查詢列的子查詢
-- 在這個例子中,子查詢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. 子查詢的優化
- MySQL5.6之前
- 在
MySQL5.6
之前,優化器會把子查詢重寫成exists
的形式
select a from t1 where a in (select a from t2); -- 這個是一條獨立的子查詢,時間復雜度 O(M+N) -- -- 經過優化器重寫後 -- select a from t1 where exists (select 1 from t2 where t1.a = t2.a); -- 這是相關子查詢,復雜度O(M*N + M)
- 在
MySQL 5.6
之前,部分的子查詢需要重寫成join的形式 (註意表的大小)
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之後
在MySQL 5.6
之後,優化器不會
將子查詢重寫
成exists
的形式,而是自動優化,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數據庫的
比較操作
,除了返回1(True)
,0(False)
之外,還會返回NULL
NULL
和NULL
的比較,返回的還是NULL
mysql> select null not in (‘a‘, ‘b‘, null);
+------------------------------+
| null not in (‘a‘, ‘b‘, null) |
+------------------------------+
| NULL | -- null不在(‘a‘, ‘b‘, null)中,返回的還是null,因為有null和null的比較
+------------------------------+
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 | -- 這個返回值可以理解 ‘c‘不在(‘a‘, ‘b‘)中,返回1,即為True
+-----------------------+
1 row in set (0.00 sec)
mysql> select ‘c‘ not in (‘a‘, ‘b‘, null);
+-----------------------------+
| ‘c‘ not in (‘a‘, ‘b‘, null) |
+-----------------------------+
| NULL | -- 理論上應該是返回1,即True的。但是包含了null值。則返回null
+-----------------------------+
1 row in set (0.00 sec)
對於包含了
NULL
值的IN
操作,總是返回True
或者NULL
NOT IN
返回NOT True (False)
或者NOT NULL (NULL)
--
-- SQL語句一 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語句二 使用 IN
--
select customerid, companyname
from customers as A
where country = ‘Spain‘
and customerid not in (select customerid from orders);
-----
-- 當結果集合中沒有NULL值時,上述兩條SQL語句查詢的結果是一致的
-----
--
-- 插入一個NULL值
--
insert into orders(orderid) values (null);
-----
-- SQL語句1 : 返回和之前一致
-- SQL語句2 : 返回為空表,因為子查詢返回的結果集中存在NULL值。not in null 永遠返回False或者NULL
-- 此時 where (country = ‘Spain‘ and (False or NULL)) 為 False OR NULL,條件永遠不匹配
-----
--
-- SQL語句2 改寫後
--
select customerid, companyname
from customers as A
where country = ‘Spain‘
and customerid not in (select customerid from orders
where customerid is not null); -- 增加這個過濾條件,使用is not,而不是<>
--
-- 和 null比較,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
不管返回值是什麽,而是看是否有行
返回,所以EXISTS
中子查詢都是select *
、select 1
等,因為只關心返回是否有行(結果集)
二. INSERT
官方文檔
(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); -- 插入一個值
Query OK, 1 row affected (0.01 sec)
(gcdb@localhost) 10:46:44 [mytest]> insert into t1(a) values(9),(11); -- 插入多個值,MySQL獨有
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 語法,MySQ獨有
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); -- 有多個列
Query OK, 0 rows affected (0.15 sec)
(gcdb@localhost) 10:47:24 [mytest]> insert into t3 select 12; -- 沒有指定列,報錯
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個數和類型
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; -- 從t3表中查詢數據並插回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; -- 從t2表中查詢數據並插入到t3(a)中,註意指定列
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)
--
-- 如果想快速增長表格中的數據,可以使用如下方法,使得數據成倍增長
--
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
官方文檔
(gcdb@localhost) 11:02:10 [mytest]> delete from t3 where b is null; -- 根據過濾條件刪除
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; -- 刪除整個表
Query OK, 2 rows affected (0.00 sec)
(gcdb@localhost) 11:03:17 [mytest]> select * from t3;
Empty set (0.00 sec)
四. UPDATE
官方文檔
(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)
--
-- 關聯後更新
--
(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的結果然後將結果集中的t2.a設置為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 | -- 該行原先2被更新成100
| 4 |
| 8 |
| 1000 | -- 該行原先10被更新成100
+------+
4 rows in set (0.01 sec)
五. REPLACE
官方文檔
(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,主鍵值1,重復
ERROR 1062 (23000): Duplicate entry ‘1‘ for key ‘PRIMARY‘
(gcdb@localhost) 11:20:28 [mytest]> replace into t4 values(1,10); -- 替換該主鍵對應的值
Query OK, 2 rows affected (0.00 sec) -- 兩行記錄受到影響
(gcdb@localhost) 11:20:54 [mytest]> select * from t4;
+---+------+
| a | b |
+---+------+
| 1 | 10 | -- 已經被更新
| 2 | 2 |
| 3 | 3 |
+---+------+
3 rows in set (0.00 sec)
-----
-- replace的原理是:先delete,在insert ;註意:需要delete和insert權限
-----
(gcdb@localhost) 11:24:23 [mytest]> replace into t4 values(5,15); -- 沒有替換對象時,類似插入效果
Query OK, 1 row affected (0.03 sec) -- 只影響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原理更明顯的例子
--
(gcdb@localhost) 11:24:35 [mytest]> create table t6
-> (a int primary key,
-> b int auto_increment, -- b是auto_increment的int型數據
-> 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類為自增列
+---+---+------+
| 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 | --當a=1時,c列值10被替換為99和b列值3自增加1為4
| 2 | 2 | 20 |
| 3 | 3 | 30 |
+---+---+------+
3 rows in set (0.00 sec)
-----
--
-- insert on duplicate 效果和 replace類似
--
(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); -- 插入報錯,存在key為1的記錄
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; -- 帶上on duplicate參數(非SQL標準,不推薦)
Query OK, 2 rows affected (0.00 sec)
(gcdb@localhost) 11:34:31 [mytest]> select * from t4;
+---+------+
| a | b |
+---+------+
| 1 | 1 | - 該行的b列從10被替換成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); -- 忽略重復的錯誤
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. 關聯更新和行號查詢
- 關聯更新
(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
- 顯示行號(RowNumber)
--
-- 方法一
--
(gcdb@localhost) 11:41:08 [mytest]> use employees;
Database changed
(gcdb@localhost) 11:42:15 [employees]> set @rn:=0; -- 產生 SESSION(會話)級別的變量
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 是賦值的意思
+-----------+--------+--------+
| 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 (推薦)
--
(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 自定義變量,根據每一記錄進行變化的
(gcdb@localhost) 11:44:55 [employees]> select @rn1:=0;
+---------+
| @rn1:=0 |
+---------+
| 0 | -- 只有一行記錄
+---------+
1 row in set (0.00 sec)
-- 相當於 把 employees 和 (select @rn1:=0)做了笛卡爾積,然後使用@rn1:=@rn + 1,根據每行進行累加
--
-- ":=" 和 "="
--
(gcdb@localhost) 11:42:15 [employees]> set @rn:=0; -- 賦值為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; -- 賦值為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; -- 進行比較
+-------+
| @a=99 |
+-------+
| 0 |
+-------+
1 row in set (0.00 sec)
- 使用子查詢實現RowNumber
- 思路
- 假設當前在第N行記錄,通過主鍵emp_no遍歷有多少行的記錄
小於等於
當前行,即為當前行的行數
- 假設當前在第N行記錄,通過主鍵emp_no遍歷有多少行的記錄
SQL語句
(gcdb@localhost) 12:07:39 [employees]> SELECT (SELECT COUNT(1) FROM employees b WHERE b.emp_no <= a.emmp_no ) AS row_number, emp_no,CONCAT(last_name," ",first_name) name,gender,hire_date FROM employees aORDER 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) -- 假設當前在第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; --查找小於等於5的行數有幾行 +----------+ | count(*) | +----------+ | 5 | -- 小於等於10005的記錄有5行,則5就是10005該行記錄的行號 +----------+ 1 row in set (0.00 sec) -- 將該子查詢的結果即可作為RowNumber,子查詢循環多次,不推薦使用。 -- 推薦使用下面這種方法 (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)
012:子查詢和增刪改查