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: 0
mysql> 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,'張國榮'),(1010,19,'梁朝偉');
Query OK, 5 rows affected (0.00 sec)
Records: 5 Duplicates: 0 Warnings: 0
mysql> select * from stu;
+------+-----+---------------+
| id | age | name |
+------+-----+---------------+
| 1001 | 18 | 路人甲Java |
| 1003 | 18 | 張學(xué)友 |
| 1004 | 20 | 張國榮 |
| 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','張國榮'),(1010,'1962-05-16','梁朝偉');
Query OK, 5 rows affected (0.00 sec)
Records: 5 Duplicates: 0 Warnings: 0
mysql>
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: 0
mysql> 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: 0
mysql> 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)
分頁查詢
開發(fā)過程中，分頁我們經(jīng)常使用，分頁一般有2個(gè)參數(shù)：
page：表示第幾頁，從1開始，范圍[1,+∞)
pageSize：每頁顯示多少條記錄，范圍[1,+∞)
如：page = 2，pageSize = 10，表示獲取第2頁10條數(shù)據(jù)。
我們使用limit實(shí)現(xiàn)分頁，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，每頁顯示2條，依次獲取所有訂單數(shù)據(jù)、第1頁、第2頁、第3頁數(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 1
mysql> 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 1
mysql>
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 1
mysql> 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 1
mysql> 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: 0
mysql> 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2條數(shù)據(jù)，來獲取數(shù)據(jù)。
下面的sql依次為第1頁、第2頁、第3頁、第4頁、第5頁的數(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頁和第3頁的數(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，分頁數(shù)據(jù)都正常了，第5頁也沒有數(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)致分頁結(jié)果亂序，可以在后面追加一個(gè)主鍵排序
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