SQL命令

Posted 2020-12-31 cyrgrowing1215

一，数据库查询

(1)格式：
SELECT <列名1,2,3...> FROM <表名> [WHERE子句] [GROUP BY 子句] [HAVING 子句] [ORDER BY子句] [LIMIT子句];
例子：
SELECT 1+4-3;
SELECT * FROM table;
SELECT a,b FROM table;
SELECT a,b AS ‘别名‘ FROM table;(别名不允许出现在WHERE中)
SELECT a,b,b+100 FROM table;

//case条件语句
SELECT a,
CASE
WHEN sex=‘0‘ THEN ‘男‘
ELSE ‘女‘
END AS ‘性别‘
FROM table;

//聚合函数 COUNT,MAX,MIN,SUM,AVG,STD,VARIANCE...
SELECT MAX(a) FROM table;

//连接表(JOIN)
交叉连接（CROSS JOIN）:SELECT * FROM a CROSS JOIN b;
内连接（INNER JOIN）:SELECT * FROM a INNER JOIN b ON a.id>b.id;有限制的交叉连接
相等连接：上面a.id=b.id;
不等连接：类似上面
自然连接：(NATURAL JOIN)

//WHERE子句
比较运算：SELECT * FROM table WHERE a>2;
字符串统配:SELECET id,name FROM table WHERE name LIKE ‘李%‘ 百分号不匹配NULL，匹配0,1，多个字符，下划线匹配一个字符；
文本匹配（正则表达式REGEXP）:SELECT name,age,major FROM table WHERE major REGEXP ‘物‘;//匹配专业里面含有物字的所有行 （| = OR）([ ] = 集合) (=转义字符） （*,+,?,{n},{n,},{n,m} = 重复元字符）（^,&=匹配定位符）
判定范围：
1) BETWEEN AND
SELECE name,age FROM table WHERE age BETWEEN 20 AND 23;
2) IN
SELECT ....
WHERE age IN (20,23);
判定空值(IS [NOT] NULL):SELECT a,b FROM table WHERE b IS NULL;

//GROUP BY 分组
GROUP BY { <列名> | <表达式> |<位置>} [ASC | DESC]]
SELECT sex,count(*) AS ‘人数‘ FROM table GROUP BY sex;
SELECT maior AVG(age) AS ‘平均年龄‘ FROM table GROUP BY major;

//HAVING 过滤分组
SELECT major,COUNT() AS ‘总人数‘ FROM table GROUP BY major HAVING COUNT()>2;
//ORDER BY 同GROUP BY
SELECT name,age FORM table ORDER BY age DESC;//NULL排序为最小值

//LIMIT 限制显示行数
SELECT id,name FROM table LIMIT 4,3;//第五行开始取三行数据
SELECT id,name FROM table ORDER BY id LIMIT 3 OFFSET 4;//同上

//UNION联合查询(多条SELECT联合查询，返回一条查询结果，ORDER,LIMIT必须置于最后一个SELECT)
SELECT... UNION [ALL |DISTINCT] SELECT...

SELECT name,major,sex FROM table WHERE majot=‘物理专业‘ UNION SELECT name,.... FROM table WHERE sex=‘1‘;

二，索引
创建索引：
1)CREATE INDEX <索引名> ON table( <列名>[ <长度> ] [ASC| DESC] )
CREATE INDEX index_name ON table(a(3) ASC);//单列索引
CREATE INDEX index_name ON table(a,b) USING BTREE;//组合索引
2)CREATE TABLE table (id,INT,NOT NULL,........,
PRIMARY KEY(id), //主键
INDEX indx_course(name), //普通索引
or
UNIQUE INDEX index_name(a) //唯一索引
or
FOREIGN KEY index_name(a))；//外键
3）ALTE TABLE table
ADD INDEX index_name(a);
or
ADD PRIMARY KEY(a);
or
ADD UNIQUE INDEX index_name(a);
or
ADD FOREIGN KEY index_name(a);
查看索引：
SHOW INDEX FROM table FROM database;
删除索引：
1)DROP INDEX index_name ON table;
2)ALTER TABLE table
DROP PRIMARY KEY(a);
or
DROP INDEX index_name(a);
or
DROP FORIEN KEY index_name(a);
三，视图（虚拟表，不包含数据，仅显示数据）
创建和删除视图
CREATE VIEW <视图名> AS

CREATE OR REPLACE VIEW view_name
AS
SELECT a,b,c
FROM table
WHERE a =‘0‘
WITH CHECK OPTION ;//(插入数据需符合WHERE条件)

DROP VIEW view_name;

修改查看视图：
ALTER VIEW view_name AS

SHOW CREAE VIEW view_name

更新视图数据（插入，修改，删除更新视图时实际更新了基础表数据,视图更新受限）
INSERT INTO view_name VALUES(.....)；//视图依赖多个基础表时，不能插入，更新只更新一个表，不能删除
UPDATE view_name SET a = ‘0‘ WHERE b = 22；
DELETE FROM view_name WHERE a=‘王五‘;

查询视图数据（同查询表）
SELECT a,b FROM view_name WHERE A=‘0‘;

创建嵌套视图
CREATE VIEW view_a AS SELECT * FROM view_b WHERE c=‘物理专业‘;