排序查詢（order by）

電商中：我們想查看今天所有成交的訂單，按照交易額從高到低排序，此時(shí)我們可以使用數(shù)據(jù)庫(kù)中的排序功能來完成。

排序語法：

select 字段名 from 表名 order by 字段1 [asc|desc],字段2 [asc|desc]; 需要排序的字段跟在order by之后； asc|desc表示排序的規(guī)則，asc：升序，desc：降序，默認(rèn)為asc； 支持多個(gè)字段進(jìn)行排序，多字段排序之間用逗號(hào)隔開。

單字段排序

mysql> create table test2(a int,b varchar(10));Query OK, 0 rows affected (0.01 sec)mysql> insert into test2 values (10,’jack’),(8,’tom’),(5,’ready’),(100,’javacode’);Query OK, 4 rows affected (0.00 sec)Records: 4 Duplicates: 0 Warnings: 0mysql> select * from test2;+------+----------+| a | b |+------+----------+| 10 | jack || 8 | tom || 5 | ready || 100 | javacode |+------+----------+4 rows in set (0.00 sec)mysql> select * from test2 order by a asc;+------+----------+| a | b |+------+----------+| 5 | ready || 8 | tom || 10 | jack || 100 | javacode |+------+----------+4 rows in set (0.00 sec)mysql> select * from test2 order by a desc;+------+----------+| a | b |+------+----------+| 100 | javacode || 10 | jack || 8 | tom || 5 | ready |+------+----------+4 rows in set (0.00 sec)mysql> select * from test2 order by a;+------+----------+| a | b |+------+----------+| 5 | ready || 8 | tom || 10 | jack || 100 | javacode |+------+----------+4 rows in set (0.00 sec)

多字段排序

比如學(xué)生表，先按學(xué)生年齡降序，年齡相同時(shí)，再按學(xué)號(hào)升序，如下：

mysql> create table stu(id int not null comment ’學(xué)號(hào)’ primary key,age tinyint not null comment ’年齡’,name varchar(16) comment ’姓名’);Query OK, 0 rows affected (0.01 sec)mysql> insert into stu (id,age,name) values (1001,18,’路人甲Java’),(1005,20,’劉德華’),(1003,18,’張學(xué)友’),(1004,20,’張國(guó)榮’),(1010,19,’梁朝偉’);Query OK, 5 rows affected (0.00 sec)Records: 5 Duplicates: 0 Warnings: 0mysql> select * from stu;+------+-----+---------------+| id | age | name |+------+-----+---------------+| 1001 | 18 | 路人甲Java || 1003 | 18 | 張學(xué)友 || 1004 | 20 | 張國(guó)榮 || 1005 | 20 | 劉德華 || 1010 | 19 | 梁朝偉 |+------+-----+---------------+5 rows in set (0.00 sec)mysql> select * from stu order by age desc,id asc;+------+-----+---------------+| id | age | name |+------+-----+---------------+| 1004 | 20 | 張國(guó)榮 || 1005 | 20 | 劉德華 || 1010 | 19 | 梁朝偉 || 1001 | 18 | 路人甲Java || 1003 | 18 | 張學(xué)友 |+------+-----+---------------+5 rows in set (0.00 sec)

按別名排序

mysql> select * from stu;+------+-----+---------------+| id | age | name |+------+-----+---------------+| 1001 | 18 | 路人甲Java || 1003 | 18 | 張學(xué)友 || 1004 | 20 | 張國(guó)榮 || 1005 | 20 | 劉德華 || 1010 | 19 | 梁朝偉 |+------+-----+---------------+5 rows in set (0.00 sec)mysql> select age ’年齡’,id as ’學(xué)號(hào)’ from stu order by 年齡 asc,學(xué)號(hào) desc;+--------+--------+| 年齡 | 學(xué)號(hào) |+--------+--------+| 18 | 1003 || 18 | 1001 || 19 | 1010 || 20 | 1005 || 20 | 1004 |+--------+--------+

按函數(shù)排序

有學(xué)生表（id：編號(hào)，birth：出生日期，name：姓名），如下：

mysql> drop table if exists student;Query OK, 0 rows affected (0.01 sec)mysql> CREATE TABLE student ( -> id int(11) NOT NULL COMMENT ’學(xué)號(hào)’, -> birth date NOT NULL COMMENT ’出生日期’, -> name varchar(16) DEFAULT NULL COMMENT ’姓名’, -> PRIMARY KEY (id) -> );Query OK, 0 rows affected (0.01 sec)mysql> insert into student (id,birth,name) values (1001,’1990-10-10’,’路人甲Java’),(1005,’1960-03-01’,’劉德華’),(1003,’1960-08-16’,’張學(xué)友’),(1004,’1968-07-01’,’張國(guó)榮’),(1010,’1962-05-16’,’梁朝偉’);Query OK, 5 rows affected (0.00 sec)Records: 5 Duplicates: 0 Warnings: 0mysql>mysql> SELECT * FROM student;+------+------------+---------------+| id | birth | name |+------+------------+---------------+| 1001 | 1990-10-10 | 路人甲Java || 1003 | 1960-08-16 | 張學(xué)友 || 1004 | 1968-07-01 | 張國(guó)榮 || 1005 | 1960-03-01 | 劉德華 || 1010 | 1962-05-16 | 梁朝偉 |+------+------------+---------------+5 rows in set (0.00 sec)

需求：按照出生年份升序、編號(hào)升序，查詢出編號(hào)、出生日期、出生年份、姓名，2種寫法如下：

mysql> SELECT id 編號(hào),birth 出生日期,year(birth) 出生年份,name 姓名 from student ORDER BY year(birth) asc,id asc;+--------+--------------+--------------+---------------+| 編號(hào) | 出生日期 | 出生年份 | 姓名 |+--------+--------------+--------------+---------------+| 1003 | 1960-08-16 | 1960 | 張學(xué)友 || 1005 | 1960-03-01 | 1960 | 劉德華 || 1010 | 1962-05-16 | 1962 | 梁朝偉 || 1004 | 1968-07-01 | 1968 | 張國(guó)榮 || 1001 | 1990-10-10 | 1990 | 路人甲Java |+--------+--------------+--------------+---------------+5 rows in set (0.00 sec)mysql> SELECT id 編號(hào),birth 出生日期,year(birth) 出生年份,name 姓名 from student ORDER BY 出生年份 asc,id asc;+--------+--------------+--------------+---------------+| 編號(hào) | 出生日期 | 出生年份 | 姓名 |+--------+--------------+--------------+---------------+| 1003 | 1960-08-16 | 1960 | 張學(xué)友 || 1005 | 1960-03-01 | 1960 | 劉德華 || 1010 | 1962-05-16 | 1962 | 梁朝偉 || 1004 | 1968-07-01 | 1968 | 張國(guó)榮 || 1001 | 1990-10-10 | 1990 | 路人甲Java |+--------+--------------+--------------+---------------+5 rows in set (0.00 sec)

說明：year函數(shù)：屬于日期函數(shù)，可以獲取對(duì)應(yīng)日期中的年份。上面使用了2種方式排序，第一種是在order by中使用了函數(shù)，第二種是使用了別名排序。

where之后進(jìn)行排序

有訂單數(shù)據(jù)如下：

mysql> drop table if exists t_order;Query OK, 0 rows affected, 1 warning (0.00 sec)mysql> create table t_order( -> id int not null auto_increment comment ’訂單編號(hào)’, -> price decimal(10,2) not null default 0 comment ’訂單金額’, -> primary key(id) -> )comment ’訂單表’;Query OK, 0 rows affected (0.01 sec)mysql> insert into t_order (price) values (88.95),(100.68),(500),(300),(20.88),(200.5);Query OK, 6 rows affected (0.00 sec)Records: 6 Duplicates: 0 Warnings: 0mysql> select * from t_order;+----+--------+| id | price |+----+--------+| 1 | 88.95 || 2 | 100.68 || 3 | 500.00 || 4 | 300.00 || 5 | 20.88 || 6 | 200.50 |+----+--------+6 rows in set (0.00 sec)

需求：查詢訂單金額>=100的，按照訂單金額降序排序，顯示2列數(shù)據(jù)，列頭：訂單編號(hào)、訂單金額，如下：

mysql> select a.id 訂單編號(hào),a.price 訂單金額 from t_order a where a.price>=100 order by a.price desc;+--------------+--------------+| 訂單編號(hào) | 訂單金額 |+--------------+--------------+| 3 | 500.00 || 4 | 300.00 || 6 | 200.50 || 2 | 100.68 |+--------------+--------------+4 rows in set (0.00 sec)

limit介紹

limit用來限制select查詢返回的行數(shù)，常用于分頁(yè)等操作。

語法：

select 列 from 表 limit [offset,] count;

說明：

offset：表示偏移量，通俗點(diǎn)講就是跳過多少行，offset可以省略，默認(rèn)為0，表示跳過0行；范圍：[0,+∞)。 count：跳過offset行之后開始取數(shù)據(jù)，取count行記錄；范圍：[0,+∞)。 limit中offset和count的值不能用表達(dá)式。

下面我們列一些常用的示例來加深理解。

獲取前n行記錄

select 列 from 表 limit 0,n;或者select 列 from 表 limit n;

示例，獲取訂單的前2條記錄，如下：

mysql> create table t_order( -> id int not null auto_increment comment ’訂單編號(hào)’, -> price decimal(10,2) not null default 0 comment ’訂單金額’, -> primary key(id) -> )comment ’訂單表’;Query OK, 0 rows affected (0.01 sec)mysql> insert into t_order (price) values (88.95),(100.68),(500),(300),(20.88),(200.5);Query OK, 6 rows affected (0.01 sec)Records: 6 Duplicates: 0 Warnings: 0mysql> select * from t_order;+----+--------+| id | price |+----+--------+| 1 | 88.95 || 2 | 100.68 || 3 | 500.00 || 4 | 300.00 || 5 | 20.88 || 6 | 200.50 |+----+--------+6 rows in set (0.00 sec)mysql> select a.id 訂單編號(hào),a.price 訂單金額 from t_order a limit 2;+--------------+--------------+| 訂單編號(hào) | 訂單金額 |+--------------+--------------+| 1 | 88.95 || 2 | 100.68 |+--------------+--------------+2 rows in set (0.00 sec)mysql> select a.id 訂單編號(hào),a.price 訂單金額 from t_order a limit 0,2;+--------------+--------------+| 訂單編號(hào) | 訂單金額 |+--------------+--------------+| 1 | 88.95 || 2 | 100.68 |+--------------+--------------+2 rows in set (0.00 sec)

獲取最大的一條記錄

我們需要獲取訂單金額最大的一條記錄，可以這么做：先按照金額降序，然后取第一條記錄，如下：

mysql> select a.id 訂單編號(hào),a.price 訂單金額 from t_order a order by a.price desc;+--------------+--------------+| 訂單編號(hào) | 訂單金額 |+--------------+--------------+| 3 | 500.00 || 4 | 300.00 || 6 | 200.50 || 2 | 100.68 || 1 | 88.95 || 5 | 20.88 |+--------------+--------------+6 rows in set (0.00 sec)mysql> select a.id 訂單編號(hào),a.price 訂單金額 from t_order a order by a.price desc limit 1;+--------------+--------------+| 訂單編號(hào) | 訂單金額 |+--------------+--------------+| 3 | 500.00 |+--------------+--------------+1 row in set (0.00 sec)mysql> select a.id 訂單編號(hào),a.price 訂單金額 from t_order a order by a.price desc limit 0,1;+--------------+--------------+| 訂單編號(hào) | 訂單金額 |+--------------+--------------+| 3 | 500.00 |+--------------+--------------+1 row in set (0.00 sec)

獲取排名第n到m的記錄

我們需要先跳過n-1條記錄，然后取m-n+1條記錄，如下：

select 列 from 表 limit n-1,m-n+1;

如：我們想獲取訂單金額最高的3到5名的記錄，我們需要跳過2條，然后獲取3條記錄，如下：

mysql> select a.id 訂單編號(hào),a.price 訂單金額 from t_order a order by a.price desc;+--------------+--------------+| 訂單編號(hào) | 訂單金額 |+--------------+--------------+| 3 | 500.00 || 4 | 300.00 || 6 | 200.50 || 2 | 100.68 || 1 | 88.95 || 5 | 20.88 |+--------------+--------------+6 rows in set (0.00 sec)mysql> select a.id 訂單編號(hào),a.price 訂單金額 from t_order a order by a.price desc limit 2,3;+--------------+--------------+| 訂單編號(hào) | 訂單金額 |+--------------+--------------+| 6 | 200.50 || 2 | 100.68 || 1 | 88.95 |+--------------+--------------+3 rows in set (0.00 sec)

分頁(yè)查詢

開發(fā)過程中，分頁(yè)我們經(jīng)常使用，分頁(yè)一般有2個(gè)參數(shù)：page：表示第幾頁(yè)，從1開始，范圍[1,+∞)pageSize：每頁(yè)顯示多少條記錄，范圍[1,+∞)如：page = 2，pageSize = 10，表示獲取第2頁(yè)10條數(shù)據(jù)。我們使用limit實(shí)現(xiàn)分頁(yè)，語法如下：

select 列 from 表名 limit (page - 1) * pageSize,pageSize;

需求：我們按照訂單金額降序，每頁(yè)顯示2條，依次獲取所有訂單數(shù)據(jù)、第1頁(yè)、第2頁(yè)、第3頁(yè)數(shù)據(jù)，如下：

mysql> select a.id 訂單編號(hào),a.price 訂單金額 from t_order a order by a.price desc;+--------------+--------------+| 訂單編號(hào) | 訂單金額 |+--------------+--------------+| 3 | 500.00 || 4 | 300.00 || 6 | 200.50 || 2 | 100.68 || 1 | 88.95 || 5 | 20.88 |+--------------+--------------+6 rows in set (0.00 sec)mysql> select a.id 訂單編號(hào),a.price 訂單金額 from t_order a order by a.price desc limit 0,2;+--------------+--------------+| 訂單編號(hào) | 訂單金額 |+--------------+--------------+| 3 | 500.00 || 4 | 300.00 |+--------------+--------------+2 rows in set (0.00 sec)mysql> select a.id 訂單編號(hào),a.price 訂單金額 from t_order a order by a.price desc limit 2,2;+--------------+--------------+| 訂單編號(hào) | 訂單金額 |+--------------+--------------+| 6 | 200.50 || 2 | 100.68 |+--------------+--------------+2 rows in set (0.00 sec)mysql> select a.id 訂單編號(hào),a.price 訂單金額 from t_order a order by a.price desc limit 4,2;+--------------+--------------+| 訂單編號(hào) | 訂單金額 |+--------------+--------------+| 1 | 88.95 || 5 | 20.88 |+--------------+--------------+2 rows in set (0.00 sec)

避免踩坑

limit中不能使用表達(dá)式

mysql> select * from t_order where limit 1,4+1;ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near ’limit 1,4+1’ at line 1mysql> select * from t_order where limit 1+0;ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near ’limit 1+0’ at line 1mysql>

結(jié)論：limit后面只能夠跟明確的數(shù)字。

limit后面的2個(gè)數(shù)字不能為負(fù)數(shù)

mysql> select * from t_order where limit -1;ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near ’limit -1’ at line 1mysql> select * from t_order where limit 0,-1;ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near ’limit 0,-1’ at line 1mysql> select * from t_order where limit -1,-1;ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near ’limit -1,-1’ at line 1

排序分頁(yè)存在的坑

準(zhǔn)備數(shù)據(jù)：

mysql> insert into test1 (b) values (1),(2),(3),(4),(2),(2),(2),(2);Query OK, 8 rows affected (0.01 sec)Records: 8 Duplicates: 0 Warnings: 0mysql> select * from test1;+---+---+| a | b |+---+---+| 1 | 1 || 2 | 2 || 3 | 3 || 4 | 4 || 5 | 2 || 6 | 2 || 7 | 2 || 8 | 2 |+---+---+8 rows in set (0.00 sec)mysql> select * from test1 order by b asc;+---+---+| a | b |+---+---+| 1 | 1 || 2 | 2 || 5 | 2 || 6 | 2 || 7 | 2 || 8 | 2 || 3 | 3 || 4 | 4 |+---+---+8 rows in set (0.00 sec)

下面我們按照b升序，每頁(yè)2條數(shù)據(jù)，來獲取數(shù)據(jù)。

下面的sql依次為第1頁(yè)、第2頁(yè)、第3頁(yè)、第4頁(yè)、第5頁(yè)的數(shù)據(jù)，如下：

mysql> select * from test1 order by b asc limit 0,2;+---+---+| a | b |+---+---+| 1 | 1 || 2 | 2 |+---+---+2 rows in set (0.00 sec)mysql> select * from test1 order by b asc limit 2,2;+---+---+| a | b |+---+---+| 8 | 2 || 6 | 2 |+---+---+2 rows in set (0.00 sec)mysql> select * from test1 order by b asc limit 4,2;+---+---+| a | b |+---+---+| 6 | 2 || 7 | 2 |+---+---+2 rows in set (0.00 sec)mysql> select * from test1 order by b asc limit 6,2;+---+---+| a | b |+---+---+| 3 | 3 || 4 | 4 |+---+---+2 rows in set (0.00 sec)mysql> select * from test1 order by b asc limit 7,2;+---+---+| a | b |+---+---+| 4 | 4 |+---+---+1 row in set (0.00 sec)

上面有2個(gè)問題：

問題1：看一下第2個(gè)sql和第3個(gè)sql，分別是第2頁(yè)和第3頁(yè)的數(shù)據(jù)，結(jié)果出現(xiàn)了相同的數(shù)據(jù)，是不是懵逼了。

問題2：整個(gè)表只有8條記錄，怎么會(huì)出現(xiàn)第5頁(yè)的數(shù)據(jù)呢，又懵逼了。

我們來分析一下上面的原因：主要是b字段存在相同的值，當(dāng)排序過程中存在相同的值時(shí)，沒有其他排序規(guī)則時(shí)，mysql懵逼了，不知道怎么排序了。

就像我們上學(xué)站隊(duì)一樣，按照身高排序，那身高一樣的時(shí)候如何排序呢？身高一樣的就亂排了。

建議：排序中存在相同的值時(shí)，需要再指定一個(gè)排序規(guī)則，通過這種排序規(guī)則不存在二義性，比如上面可以再加上a降序，如下：

mysql> select * from test1 order by b asc,a desc;+---+---+| a | b |+---+---+| 1 | 1 || 8 | 2 || 7 | 2 || 6 | 2 || 5 | 2 || 2 | 2 || 3 | 3 || 4 | 4 |+---+---+8 rows in set (0.00 sec)mysql> select * from test1 order by b asc,a desc limit 0,2;+---+---+| a | b |+---+---+| 1 | 1 || 8 | 2 |+---+---+2 rows in set (0.00 sec)mysql> select * from test1 order by b asc,a desc limit 2,2;+---+---+| a | b |+---+---+| 7 | 2 || 6 | 2 |+---+---+2 rows in set (0.00 sec)mysql> select * from test1 order by b asc,a desc limit 4,2;+---+---+| a | b |+---+---+| 5 | 2 || 2 | 2 |+---+---+2 rows in set (0.00 sec)mysql> select * from test1 order by b asc,a desc limit 6,2;+---+---+| a | b |+---+---+| 3 | 3 || 4 | 4 |+---+---+2 rows in set (0.00 sec)mysql> select * from test1 order by b asc,a desc limit 8,2;Empty set (0.00 sec)

看上面的結(jié)果，分頁(yè)數(shù)據(jù)都正常了，第5頁(yè)也沒有數(shù)據(jù)了。

總結(jié)

order by … [asc|desc]用于對(duì)查詢結(jié)果排序，asc：升序，desc：降序，asc|desc可以省略，默認(rèn)為asc limit用來限制查詢結(jié)果返回的行數(shù)，有2個(gè)參數(shù)（offset，count），offset：表示跳過多少行，count：表示跳過offset行之后取count行 limit中offset可以省略，默認(rèn)值為0 limit中offset 和 count都必須大于等于0 limit中offset和count的值不能用表達(dá)式 分頁(yè)排序時(shí)，排序不要有二義性，二義性情況下可能會(huì)導(dǎo)致分頁(yè)結(jié)果亂序，可以在后面追加一個(gè)主鍵排序

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