SELECT
SELECT
1. 基础(*指代表全部数据)
SELECT *
FROM;
2. 计算,伪表(DUAL)
SELECT 1+2
[FROM DUAL];
3. 查某些字段(假设id为一些字段)
SELECT ID
FROM employees;
4. 别名(AS即Alias
4.1 SELECT ID"编号"
FROM employees;
4.2 SELECT ID AS department_ID
FROM employees;
5. 去重(一般只去重一个字段的
SELECT DISTINCT ID
FROM employees;
6. 空值参与运算
一般空值参与运算结果为null;
SELECT IFNULL(1+2,0)
FROM DUAL;
7. 查询表结构
DESC employees;
8. 查询常量(将在前面加上
SELECT 'SHANGGUIGU',ID
FROM employees;
9. 过滤(WHERE
SELECT *
FROM employees
WHERE id = 1001;