2024年4月2日发(作者：)

sql语句学习

数据定义语言（DDL）：

1）创建数据库（create）：create database database-name;

eg. create database test;

2）删除数据库：drop database dbname;

database test;

3）创建新表：create table tabname(col1 type1 [not null] [primary key],col2

type2 [not null],..);

eg.根据已有的表创建新表的例子：create table tab_new like tab_old；create

table tab_new as select col1,col2… from tab_old definition only;

4）删除表：drop table tabname;

5）增加列：alter table tabname add column col type;

6）添加主键： alter table tabname add primary key(col) ;

7）删除主键：alter table tabname drop primary key(col) ;

8）创建索引：create [unique] index idxname on tabname(col….) ;

9）删除索引：drop index idxname; 注：索引是不可更改的，想更改必须删除重

新建；

10）创建视图：create view viewname as select statement;

2. 数据操纵语言（DML）

1）查询语句（select）

eg1. select * from table1 where field1 like '%value1%';

eg2. select * from table1 order by field1,field2 [desc];

eg3. select count as totalcount from table1;

eg4. select sum(field1) as sumvalue from table1;

eg5. select avg(field1) as avgvalue from table1;

eg6. select max(field1) as maxvalue from table1;

eg7. select min(field1) as minvalue from table1;

eg8. select a.a, a.b, a.c, b.c, b.d, b.f from a LEFT OUT JOIN b ON a.a = b.c;（注：