select查询语句

显示表结构(DESCRIBE/DESC)

DESCRIBE employees;
DESC employees;

列的别名(AS)

SELECT employee_id emp_id FROM employees; #省略写法
SELECT employee_id AS emp_id FROM employees; #AS未省略写法
SELECT employee_id "emp_id" FROM employees; #""双引号写法，适用于别名有空格的

去重(DISTINCT)　　

SELECT DISTINCT department_id FROM employees; #正确写法
SELECT salary,DISTINCT department_id FROM employees; #错误写法
SELECT DISTINCT salary,department_id FROM employees; #正确写法（同时满足salary,department_id）　

查询常数：

SELECT '测试',123,employee_id,last_name FROM employees;