SQL 语句语法简介（一）

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;