MySQL的查詢語句--SELECT

 本來想總結高可用集羣的另外幾個實驗呢，回頭看看別人總結的內容，好精細，而且擴展了好多內容，慚愧的不行，還是先跳過了，呵呵~~~

這裏先來把mysql查詢語句綜合了一下：

 

上一篇

  http://90112526.blog.51cto.com/6013499/1037266

介紹了簡單的數據庫操作等，接下來從細節入手，來介紹mysql的查詢語句；

在這裏導入了一個jiaowu數據庫，來實現以下例題的操作：

先來看下這個數據庫所包含的內容

mysql> show databases;
+--------------------+
| Database           |
+--------------------+
| information_schema |
| jiaowu             |
| mysql              |
| test               |
+--------------------+
4 rows in set (0.00 sec)

l> use jiaowu;

Database changed

mysql> show tables;
+------------------+
| Tables_in_jiaowu |
+------------------+
| courses          |
| scores           |
| students         |
| tutors           |
+------------------+
4 rows in set (0.00 sec)

mysql> select * from students;
+-----+--------------+------+--------+------+------+------+---------------------+
| SID | Name         | Age  | Gender | CID1 | CID2 | TID  | CreateTime          |
+-----+--------------+------+--------+------+------+------+---------------------+
|   1 | GuoJing      |   19 | M      |    2 |    7 |    3 | 2012-04-06 10:00:00 |
|   2 | YangGuo      |   17 | M      |    2 |    3 |    1 | 2012-04-06 10:00:00 |
|   3 | DingDian     |   25 | M      |    6 |    1 |    7 | 2012-04-06 10:00:00 |
|   4 | HuFei        |   31 | M      |    8 |   10 |    5 | 2012-04-06 10:00:00 |
|   5 | HuangRong    |   16 | F      |    5 |    9 |    9 | 2012-04-06 10:00:00 |
|   6 | YueLingshang |   18 | F      |    8 |    4 | NULL | 2012-04-06 10:00:00 |
|   7 | ZhangWuji    |   20 | M      |    1 |    7 | NULL | 2012-04-06 10:00:00 |
|   8 | Xuzhu        |   26 | M      |    2 |    4 | NULL | 2012-04-06 10:00:00 |
|   9 | LingHuchong  |   22 | M      |   11 | NULL | NULL | 2012-04-06 10:00:00 |
|  10 | YiLin        |   19 | F      |   18 | NULL | NULL | 2012-04-06 10:00:00 |
+-----+--------------+------+--------+------+------+------+---------------------+
10 rows in set (0.00 sec)

mysql> select * from tutors;
+-----+--------------+--------+------+
| TID | Tname        | Gender | Age  |
+-----+--------------+--------+------+
|   1   2 | HuangYaoshi  | M      |   63 |
|   3 | Miejueshitai | F      |   72 |
|   4 | OuYangfeng   | M      |   76 |
|   5 | YiDeng       | M      |   90 |
|   6 | YuCanghai    | M      |   56 |
|   7 | Jinlunfawang | M      |   67 |
|   8 | HuYidao      | M      |   42 |
|   9 | NingZhongze  | F      |   49 |
+-----+--------------+--------+------+
9 rows in set (0.00 sec)

這是以下例題中會用到的數據，可以先參考下；

首先是mysql查詢語句：

查詢的分類：

單表查詢：簡單查詢

多表查詢：聯結查詢

子查詢：複雜查詢

聯合查詢

 

select語句：

常用函數：

##field--表示字段

count(*) 總行數

mysql> select count(*) from tutors;
+----------+
| count(*) |
+----------+
|        9 |
+---------+
1 row in set (0.00 sec)

max(field) 返回最大值

mysql> select max(age) from tutors;
+----------+
| max(age) |
+----------+
|       93 |
+----------+
1 row in set (0.00 sec)

min(field) 返回最小值

avg(field) 平均值

mysql> select avg(age) from tutors;
+----------+
| avg(age) |
+----------+
|  67.5556 |
+----------+
1 row in set (0.00 sec)

sum() 記和

mysql> select sum(1+2);
+----------+
| sum(1+2) |
+----------+
|        3 |
+----------+
1 row in set (0.01 sec)

select 是挑選列的，where是挑選行的，二者結合起來纔是將一個實體的屬性整體顯示出來

where 後面指定的是條件：

可以指定的條件有：

算術比較：

> , < , = , ！,>= , <=, <=> （取得的結果是空值也不會出錯）

mysql> select name,age from students where age>=20;
+-------------+------+
| name        | age  |
+-------------+------+
| DingDian    |   25 |
| HuFei       |   31 |
| ZhangWuji   |   20 |
| Xuzhu       |   26 |
| LingHuchong |   22 |
+-------------+------+
5 rows in set (0.00 sec)

組合邏輯比較：

and

or

not(!)

mysql> select name,age from students where ! (age <=25);
+-------+------+
| name  | age  |
+-------+------+
| HuFei |   31 |
| Xuzhu |   26 |
+-------+------+
2 rows in set (0.00 sec)

其他條件比較：

beween …… and ……

mysql> select name,age from students where age between 24 and 30
+----------+------+
| name     | age  |
+----------+------+
| DingDian |   25 |
| Xuzhu    |   26 |
+----------+------+
2 rows in set (0.00 sec)

in 查詢的字段在指定的列表中

mysql> select name,age from students where age in (18,20,25);
+--------------+------+
| name         | age  |
+--------------+------+
| DingDian     |   25 |
| YueLingshang |   18 |
| ZhangWuji    |   20 |
+--------------+------+
3 rows in set (0.01 sec)

is null：查詢是空值的

mysql> select name from students where cid2 is null;
+-------------+
| name        |
+-------------+
| LingHuchong |
| YiLin       |
+-------------+
2 rows in set (0.00 sec)

is not null

like : 做通配符的匹配

%：匹配任意長度的任意字符

_: 匹配單個字符

regexp|rlike : 正則表達式的匹配

order by: 排序，默認是升序的asc

desc：降序

mysql> select name,age from students where age in (22,18,25)order by age desc;
+--------------+------+
| name         | age  |
+--------------+------+
| DingDian     |   25 |
| LingHuchong  |   22 |
| YueLingshang |   18 |
+--------------+------+
3 rows in set (0.00 sec)

distinct: 顯示結果的唯一性，附在select之後（以下面的例子解說，cid1相同的只顯示了一次）

mysql> select distinct cid1 from students order by cid1 desc;
+------+
| cid1 |
+------+
|   18 |
|   11 |
|    8 |
|    6 |
|    5 |
|    2 |
|    1 |
+------+
7 rows in set (0.00 sec)

group by: 將取得的結果進行分組，通常分組的結果是用來做聚合運算的

having: 對分組的結果進行條件過濾

mysql> select avg(age),cid1 from students group by cid1;
+----------+------+
| avg(age) | cid1 |
+----------+------+
|  20.0000 |    1 |
|  20.6667 |    2 |
|  16.0000 |    5 |
|  25.0000 |    6 |
|  24.5000 |    8 |
|  22.0000 |   11 |
|  19.0000 |   18 |
+----------+------+
7 rows in set (0.00 sec)

limit：限定顯示的行數

eg：limit 1,2;表示跳過第一行再顯示兩行

mysql> select avg(age),cid1 from students group by cid1 limit 1,2;
+----------+------+
| avg(age) | cid1 |
+----------+------+
|  20.6667 |    2 |
|  16.0000 |    5 |
+----------+------+
2 rows in set (0.00 sec)

聯結查詢：表和表之間建立關係

交叉聯結：又稱之爲笛卡爾乘積，它會把你所查詢的表中字段結合起來一塊顯示出來

select * from students,tutors; ##這個數據量有點大，就不在這裏貼出來了

內聯結：又叫等值聯結，在表中可以建立等值關係

mysql> select students.name,tutors.tname from students,tutors where students.TID=tutors.TID;
+-----------+--------------+
| name      | tname        |
+-----------+--------------+
| GuoJing   | Miejueshitai |
| YangGuo   | HongQigong   |
| DingDian  | Jinlunfawang |
| HuFei     | YiDeng       |
| HuangRong | NingZhongze  |
+-----------+--------------+
5 rows in set (0.00 sec)

外聯結：

左外聯結：以第一張表爲準顯示所有

tb1 LEFT JOIN tb2 ON condition

右外聯結：以第二張表爲準顯示所有

tb1 RIGHT JOIN tb2 ON condition 

select * from students LEFT JOIN tutors ON students.TID=tutors.TID;

select * from students RIGHT JOIN tutors ON students.TID=tutors.TID;

##自己執行後對比一下就可以知道了哦~~

自聯結：通過一張表建立等值關係的聯結，它是內聯結的特殊形式，可以通過給自己的表取別名進行查詢

mysql> select s.name AS students,t.name AS tutors from students AS s ,students AS t where s.TID=t.SID;
+-----------+-------------+
| students  | tutors      |
+-----------+-------------+
| GuoJing   | DingDian    |
| YangGuo   | GuoJing     |
| DingDian  | ZhangWuji   |
| HuFei     | HuangRong   |
| HuangRong | LingHuchong |
+-----------+-------------+
5 rows in set (0.00 sec)

聯合查詢：將2個select語句合併起來，關鍵字 UNION

 

mysql> select name ,age,gender from students union select tname,age,gender from tutors order by age;
+--------------+------+--------+
| name         | age  | gender |
+-------------+------+--------+
| HuangRong    |   16 | F      |
| YangGuo      |   17 | M      |
| YueLingshang |   18 | F      |
| YiLin        |   19 | F      |
| GuoJing      |   19 | M      |
| ZhangWuji    |   20 | M      |
………………………………
##下面還有哦，就不顯示了。。

子查詢：查詢語句中嵌套了別的查詢語句

分爲3類：

where子句或having子句中的子查詢

用於比較操作符合邏輯操作符中：限定子查詢的返回值只能是一行

mysql> select name,age from students where age > (select avg(age) from students);
+-------------+------+
| name        | age  |
+-------------+------+
| DingDian    |   25 |
| HuFei       |   31 |
| Xuzhu       |   26 |
| LingHuchong |   22 |
+-------------+------+
4 rows in set (0.00 sec)

               用於IN操作符中：子查詢呢的返回值可以是多行

mysql> select name,age from students where tid in (select tid from tutors where age >50);
+----------+------+
| name     | age  |
+----------+------+
| GuoJing  |   19 |
| YangGuo  |   17 |
| DingDian |   25 |
| HuFei    |   31 |
+----------+------+
4 rows in set (0.00 sec)

用於exists：結果只要存在就可以（不涉及）

用於from子句中的子查詢

 mysql> select name,age from (select name,age,gender from students as s union select tname,age,gender from tutors as t) as vt where age > 30;
+--------------+------+
| name         | age  |
+--------------+------+
| HuFei        |   31 |
| HongQigong   |   93 |
| HuangYaoshi  |   63 |
| Miejueshitai |   72 |
…………………………

用於update或delete語句中的子查詢

mysql> delete from students where age+20< (select min(age) from tutors);Query OK, 6 rows affected (0.01 sec)
mysql> select * from students;
+-----+-------------+------+--------+------+------+------+---------------------+
| SID | Name        | Age  | Gender | CID1 | CID2 | TID  | CreateTime          |
+-----+-------------+------+--------+------+------+------+---------------------+
|   3 | DingDian    |   25 | M      |    6 |    1 |    7 | 2012-04-06 10:00:00 |
|   4 | HuFei       |   31 | M      |    8 |   10 |    5 | 2012-04-06 10:00:00 |
|   8 | Xuzhu       |   26 | M      |    2 |    4 | NULL | 2012-04-06 10:00:00 |
|   9 | LingHuchong |   22 | M      |   11 | NULL | NULL | 2012-04-06 10:00:00 |
+-----+-------------+------+--------+------+------+------+---------------------+
4 rows in set (0.00 sec)

 以上主要是用例題分析了下select語句的使用，簡單的select操作希望你可以根據此能夠實現^_^~~