数据库基础
关键词:
数据库
表
(表名唯一,取决多个因素,如不同数据库的表可以同名)
模式
(关于数据库和表的布局及特性的信息)
列
(表中的字段)
行
[行(raw)和记录(record)很大程度可以等同,但行才是正确的术语]
数据类型
(限制数据种类,帮助正确排序,磁盘优化方面的作用)
主键
(primary key): 一列,其值可以唯一区分表中的行。
SQL
[(Structured Query Language):结构化查询语言。]
主键条件:
- 每行都应有一个主键,所以其值不为null。
- 任意两行间的主键值不同。
主键通常是一列,但也可多列共同构成主键。
主键设置建议:
- 不更新主键列中的值;
- 不重用主键列的值;
- 不在主键列中使用可能会更改的值。
SQL命令执行:
- 命令在
mysql>
之后输入; - 命令用
;
或\g
结束,仅按Enter
不执行命令; - 输入
help
或\h
获取帮助; - 输入
quit
或exit
退出程序。
基本语句:
myslq -u root -p;
use Database;
#SHOW相关
SHOW databases;
SHOW tables;
SHOW columns FROM tables; -- 等于 describe "tables";
SHOW CREATE DATABASE db_name; 显示完整的建库语句
SHOW CREATE TABLE tbl_name;
SHOW [STORAGE] ENGINES
#SELECT相关
SELECT column_name1,column_name2 FROM table;
SELECT *FROM tables;
-- Distinct
-- 不能部分使用DISTINCT,其应用于所有列而不是其前置列
SELECT DISTINCT column_namw FROM table;
-- Limit 从第零个开始后的5个 取的时候排列顺序是从零开始的。
SELECT column_name FROM table_name LIMIT 5;
-- 从第二个开始后的5个
SELECT column_name FROM table_name LIMIT 2,5;
-- OFFSET 限制两个,从第三为开始取
SELECT column_name FROM table_name LIMIT 2 OFFSET 3;
-- 使用全限定的表名 库:manxc 表:tags
SELECT tags.tid FROM manxc.tags;
排序检索数据
关键字:ORDER BY
SELECT column_name FROM table_name ORDER BY column_name;
默认升序,字母按A-Z排,数字从小到大;
注:排序中文时出现问题。
解决:https://www.jianshu.com/p/8a9135f9cd47?utm_campaign
升序(默认):ASC 降序:DESC
过滤数据
关键字:WHERE
(同时可与其它关键字组合)
SELECT * FROM manxc.tags WHERE tags.tid BETWEEN 2 AND 9 ORDER BY tid DESC,tagname;
操作符 | 说明 |
---|---|
= | 等于 |
<> | 不等于 |
!= | 不等于 |
< | 小于 |
<= | 小于等于 |
> | 大于 |
>= | 大于等于 |
BETWEEN | 在指定的两个值之间 |
eg:
mysql> SELECT 2 BETWEEN 1 AND 3, 2 BETWEEN 3 and 1;
-> 1, 0
mysql> SELECT 1 BETWEEN 2 AND 3;
-> 0
mysql> SELECT 'b' BETWEEN 'a' AND 'c';
-> 1
mysql> SELECT 2 BETWEEN 2 AND '3';
-> 1
mysql> SELECT 2 BETWEEN 2 AND 'x-3';
-> 0
WHERE 匹配字符加‘’;且其在执行匹配时默认不区分大小写;
mysql> SELECT uid,username,state FROM manxc.user WHERE username='FLY';
+-----+----------+-------+
| uid | username | state |
+-----+----------+-------+
| 3 | fly | 0 |
+-----+----------+-------+
空值检查:IS NULL (空值是无值和0和空格不同)
mysql> SELECT uid,username,state FROM user WHERE password IS NULL LIMIT 3;
+-----+----------+-------+
| uid | username | state |
+-----+----------+-------+
| 8 | dfdg | NULL |
| 9 | dgdg | NULL |
| 10 | gdg | NULL |
+-----+----------+-------+
数据过滤
关键词
操作符(operator)
:用来联结或改变where子句的关键字。
AND
操作符
mysql> SELECT uid,username,state FROM USER WHERE state IS NULL AND uid <= 13;
+-----+-----------+-------+
| uid | username | state |
+-----+-----------+-------+
| 8 | dfdg | NULL |
| 9 | dgdg | NULL |
| 10 | gdg | NULL |
| 11 | dgdgh | NULL |
| 12 | dgklds | NULL |
| 13 | dgkljdlkg | NULL |
+-----+-----------+-------+
OR
操作符:
mysql> SELECT uid,username,state FROM USER WHERE state IS NULL OR uid <= 13;
+-----+--------------+-------+
| uid | username | state |
+-----+--------------+-------+
| 4 | test1 | 0 |
| 3 | fly | 0 |
| 5 | test2 | 0 |
| 6 | test3 | 1 |
| 7 | 1 | 1 |
| 8 | dfdg | NULL |
| 9 | dgdg | NULL |
| 10 | gdg | NULL |
| 11 | dgdgh | NULL |
| 12 | dgklds | NULL |
| 13 | dgkljdlkg | NULL |
| 14 | fdjwe | NULL |
| 15 | gkdlkg | NULL |
| 16 | dgdlkjg | NULL |
| 17 | fdglkdjg | NULL |
| 18 | gkldssjgdsas | NULL |
| 19 | dgjkljg | NULL |
| 20 | djglkdg | NULL |
| 21 | kgdlksgj | NULL |
+-----+--------------+-------+
混合使用时的顺序:
在有多个or和and同时使用时,优先处理and,可以使用()提高优先级。
mysql> SELECT uid,username,state FROM USER WHERE (state IS NULL OR state =1) AND uid <=10;
+-----+----------+-------+
| uid | username | state |
+-----+----------+-------+
| 6 | test3 | 1 |
| 7 | 1 | 1 |
| 8 | dfdg | NULL |
| 9 | dgdg | NULL |
| 10 | gdg | NULL |
+-----+----------+-------+
5 rows in set (0.00 sec)
mysql> SELECT uid,username,state FROM USER WHERE state IS NULL OR state =1 AND uid <=10;
+-----+--------------+-------+
| uid | username | state |
+-----+--------------+-------+
| 6 | test3 | 1 |
| 7 | 1 | 1 |
| 8 | dfdg | NULL |
| 9 | dgdg | NULL |
| 10 | gdg | NULL |
| 11 | dgdgh | NULL |
| 12 | dgklds | NULL |
| 13 | dgkljdlkg | NULL |
| 14 | fdjwe | NULL |
| 15 | gkdlkg | NULL |
| 16 | dgdlkjg | NULL |
| 17 | fdglkdjg | NULL |
| 18 | gkldssjgdsas | NULL |
| 19 | dgjkljg | NULL |
| 20 | djglkdg | NULL |
| 21 | kgdlksgj | NULL |
+-----+--------------+-------+
16 rows in set (0.00 sec)
建议:使用具有AND和OR操作符的WHERE子句,都应该使用圆括号明确的分组,不用过分依赖计算次序,使用括号没有坏处且能消除歧义。
IN
操作符:
where子句使用in操作符
mysql> SELECT uid,username,state FROM user WHERE state IN (0,1);
+-----+----------+-------+
| uid | username | state |
+-----+----------+-------+
| 4 | test1 | 0 |
| 3 | fly | 0 |
| 5 | test2 | 0 |
| 6 | test3 | 1 |
| 7 | 1 | 1 |
+-----+----------+-------+
5 rows in set (0.00 sec)
IN
和 OR
有类似作用,此句表示查询state是0或1的。
IN操作符的优点:
- 使用IN时,计算次序更容易管理(操作符少了,没那么多or)
- IN操作符的语法更清楚且直观;
- IN一般比OR的执行更快;
- IN的最大优点时可以包含其它SELECT语句,使得能更动态的建立WHERE子句。
NOT
操作符:
where子句中,not用来否定之后跟的条件。
mysql> select * from tags where tid NOT IN (1,2,3,4,5,6,7,8,9,10);
+-----+---------+
| tid | tagname |
+-----+---------+
| 11 | 猎奇 |
| 12 | 少女 |
| 13 | 魔法 |
| 14 | 历史 |
| 15 | 机战 |
| 16 | 神魔 |
| 17 | 运动 |
| 18 | 励志 |
| 19 | 音乐 |
| 20 | 推理 |
| 21 | 美食 |
| 22 | 催泪 |
| 23 | 职场 |
| 26 | 搞笑 |
+-----+---------+
注:MySQL支持使用NOT
对IN
,BETWEEN
,EXISTS
子句取反。