mysql linux selected_MySQL的查询语句--SELECT
本來(lái)想總結(jié)高可用集群的另外幾個(gè)實(shí)驗(yàn)?zāi)?#xff0c;回頭看看別人總結(jié)的內(nèi)容，好精細(xì)，而且擴(kuò)展了好多內(nèi)容，慚愧的不行，還是先跳過(guò)了，呵呵~~~
介紹了簡(jiǎn)單的數(shù)據(jù)庫(kù)操作等，接下來(lái)從細(xì)節(jié)入手，來(lái)介紹mysql的查詢語(yǔ)句；
在這里導(dǎo)入了一個(gè)jiaowu數(shù)據(jù)庫(kù)，來(lái)實(shí)現(xiàn)以下例題的操作：
先來(lái)看下這個(gè)數(shù)據(jù)庫(kù)所包含的內(nèi)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(huì)用到的數(shù)據(jù)，可以先參考下；
首先是mysql查詢語(yǔ)句：
查詢的分類：
單表查詢：簡(jiǎn)單查詢
多表查詢：聯(lián)結(jié)查詢
子查詢：復(fù)雜查詢
聯(lián)合查詢
select語(yǔ)句：
常用函數(shù)：
##field--表示字段
count(*) 總行數(shù)
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(jié)合起來(lái)才是將一個(gè)實(shí)體的屬性整體顯示出來(lái)
where 后面指定的是條件：
可以指定的條件有：
算術(shù)比較：
> , < , = , ！,>= , <=, <=> (取得的結(jié)果是空值也不會(huì)出錯(cuò))
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(zhǎng)度的任意字符
_: 匹配單個(gè)字符
regexp|rlike : 正則表達(dá)式的匹配
order by: 排序，默認(rèn)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(jié)果的唯一性，附在select之后(以下面的例子解說(shuō)，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(jié)果進(jìn)行分組，通常分組的結(jié)果是用來(lái)做聚合運(yùn)算的
having: 對(duì)分組的結(jié)果進(jìn)行條件過(guò)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(shù)
eg：limit 1,2;表示跳過(guò)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)
總結(jié)
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