语句分类
SQL 命令一般分为三类:DQL、DML、DDL。
一、DDL语句。
1.1建表语句
CREATE TABLE table_name(
col01_name data_type,
col02_name data_type,
);
实例：
student_name varchar(40),
chinese_score int,
math_score int,
test_date date
);
CREATE TABLE
\d 显示表
postgres=# \d score
Table "public.score"
Column | Type | Modifiers
---------------+-----------------------+-----------
student_name | character varying(40) |
chinese_score | integer |
math_score | integer |
test_date | date |
主键
创建表的时候可以指定主键primary key，标识这一列唯一，不能重复。
postgres=# create table student(
postgres(# no int primary key,
postgres(# student_name varchar(40),
postgres(# age int);
CREATE TABLE
1.2 删除表
drop table table_name;
二、DML语句。
DML用于插入、更新和删除数据。insert、update、delete。
2.1 插入语句：
postgres=# insert into student(no,age,student_name) values(2,13,'steven');
INSERT 0 1
2.2 更新语句：
postgres=# update student set age = 15;
UPDATE 1
使用where过滤：
postgres=# update student set age=14 where no = 3;
UPDATE 1
2.3 删除语句：
postgres=# delete from student where no = 3;
DELETE 1
2.4.1 查询语句：
postgres=# select no,student_name,age from student;
no | student_name | age
----+--------------+-----
2 | haha | 15
1 | steven | 13
(2 rows)
2.4.2 过滤查询：
postgres=# select * from student where age > 14;
no | student_name | age
----+--------------+-----
2 | haha | 15
(1 row)
2.4.3排序
postgres=# select * from student order by age;
no | student_name | age
----+--------------+-----
1 | steven | 13
2 | haha | 15
(2 rows)
2.4.4倒序
postgres=# select * from student order by no desc;
no | student_name | age
----+--------------+-----
2 | haha | 15
1 | steven | 13
(2 rows)
2.4.5分组
postgres=# select age,count(*) from student group by age;
age | count
-----+-------
15 | 1
13 | 1
(2 rows)
2.4.6多表联查
postgres=# create table class(no int primary key,
class_name varchar(40));
CREATE TABLE
postgres=# insert into class values(1,'初二（1）班');
INSERT 0 1
postgres=# insert into class values(2,'初二（2）班');
INSERT 0 1
postgres=# insert into class values(3,'初二（3）班');
INSERT 0 1
postgres=# insert into class values(4,'初二（4）班');
INSERT 0 1
postgres=# select * from class;
no | class_name
----+-------------
1 | 初二（1）班
2 | 初二（2）班
3 | 初二（3）班
4 | 初二（4）班
(4 rows)
postgres=# create table student2(no int primary key,
student_name varchar(40),
age int,
class_no int);
CREATE TABLE postgres=# insert into student2 values(1,'a',12,1);
INSERT 0 1
postgres=# insert into student2 values(2,'b',13,1);
INSERT 0 1
postgres=# insert into student2 values(3,'c',15,2);
INSERT 0 1
postgres=# insert into student2 values(4,'d',15,2);
INSERT 0 1
postgres=# insert into student2 values(5,'e',15,3);
INSERT 0 1
postgres=# insert into student2 values(6,'f',15,2);
INSERT 0 1
postgres=#
postgres=# select student_name,class_name from student2,class
where student2.class_no = class.no;
student_name | class_name
--------------+-------------
a | 初二（1）班
b | 初二（1）班
c | 初二（2）班
d | 初二（2）班
e | 初二（3）班
f | 初二（2）班
(6 rows)
三、其它SQL语句
insert into ....select语句，可以把一个表插入到另外一张表中。
postgres=# create table student_bak(no int primary key,
postgres(# sutdent_name varchar(40),
postgres(# age int,
postgres(# class_no int);
CREATE TABLE
postgres=# insert into student_bak select * from student;
INSERT 0 2
postgres=# select * from student_bak;
no | sutdent_name | age | class_no
----+--------------+-----+----------
2 | haha | 15 |
1 | steven | 13 |
(2 rows)
union语句将两个表查询的数据整合在一起
postgres=# select * from student2 where no = 1 union select * from student_bak
where no =2;
no | student_name | age | class_no
----+--------------+-----+----------
1 | a | 12 | 1
2 | haha | 15 |
(2 rows)
truncate table 清楚表数据
postgres=# truncate table student_bak;