– 主键约束:PRIMARY KEY=不重复+不为NULL;
– (列级约束条件)
CREATE TABLE dept_dylan (
dept_id INT (2) PRIMARY KEY,
dept_name VARCHAR (20),
dept_location VARCHAR (40)
);
– 数据准备
INSERT INTO dept_dylan
VALUES
(10, 'developer', 'beijing');
INSERT INTO dept_dylan
VALUES
(10, 'market', 'shenzhen');
– 表级约束条件:建议约束命名规则:表名_列名_约束条件的类型
CREATE TABLE dept_dylan2 (
dept_id INT (2),
dept_name VARCHAR (20),
dept_location VARCHAR (40),
CONSTRAINT dept_dylan2_pk PRIMARY KEY (dept_id)
);
– 数据准备
INSERT INTO dept_dylan2
VALUES
(10, 'developer', 'beijing');
INSERT INTO dept_dylan2
VALUES
(10, 'market', 'shenzhen');
SELECT
*
FROM
dept_dylan2;
– 非空约束( not null , 简称 NN ) ** 非空约束只能定义在列级
CREATE TABLE student_dylan (
id INT (4) PRIMARY KEY,
NAME VARCHAR (10) NOT NULL,
age INT (3)
);
INSERT INTO student_dylan
VALUES
(1, 'zhangwei', 20);
INSERT INTO student_dylan
VALUES
(2, 'zhangwei', 19);
INSERT INTO student_dylan
VALUES
(3, NULL, 19);
SELECT
*
FROM
student_dylan;
– 给非空约束命名
DROP TABLE student_dylan;
CREATE TABLE student_dylan (
id INT (4) PRIMARY KEY,
NAME VARCHAR (10) NOT NULL,
age INT (3)
);
CREATE TABLE student_ning2 (
id INT (4),
NAME VARCHAR (10) NOT NULL,
email VARCHAR (30),
age INT (2),
CONSTRAINT student_ning2_id_pk PRIMARY KEY (id),
CONSTRAINT student_ning2_email_uk UNIQUE (email)
) ;
CREATE TABLE student_ning3 (
id INT (4) PRIMARY KEY,
NAME VARCHAR (10) NOT NULL,
email VARCHAR (30) UNIQUE,
age INT (2) CHECK (age > 10),
gender CHAR (1) CHECK (gender IN('F', 'M')) -- 'F'代表女生 ;'M'代表男生
);
– 创建视图, 视图的定义是一个数据表的子集
CREATE VIEW v_emp_dylan AS SELECT
emp_id,
NAME,
job
FROM
emp_test
WHERE
dept_test_id = 20;
DESC v_emp_dylan;
SELECT
*
FROM
v_emp_dylan;
– 创建视图 , 视图的定义是一个复杂查询语句
CREATE VIEW v_emp_count AS SELECT
emp_id,
count(*) emp_num
FROM
emp_test
GROUP BY
dept_test_id;
DESC v_emp_count;
SELECT
*
FROM
v_emp_count;
– 视图可以使用 CREATE OR REPLACE 来创建或覆盖,并可查询视图的定义。
CREATE
OR REPLACE VIEW v_emp_count AS SELECT
dept_test_id,
count(*) emp_num,
sum(salary) sum_s,
avg(IFNULL(salary, 0)) avg_s,
max(salary) max_s,
min(salary) min_s
FROM
emp_test
GROUP BY
dept_test_id;
SELECT
*
FROM
v_emp_count;
– 索引 Index *
- index :用来提高查询效率的机制
- 全表扫描方式( Full Table Scan ):查询效率极低
- 索引查询:比全表扫描快
- 索引的结构:数据+地址( 如:张三+Room203 )
- 注意:对亍数据变更频繁(DML 操作频繁)的表 , 索引会影响查询性能
- 自动创建索引:
– 如果数据表有 PK/Unique 两种约束 , 索引自动创建 , 除此以外 , 索引必须手动创建 - 自定义索引语法:
– create index 索引名 on 表名(列名) ;
– 表的主键和唯一约束条件 , 会自动创建索引
create table student_ning7(
id INT(4),
name char(20),
email char(40),
constraint stu_n7_id_pk primary key(id),
constraint stu_n7_email_uk unique(email)
)
create index idx_stu7_name
on student_ning7(name) ;
select * from student_ning7
where name = 'zhangsan' ;