进阶1 基础查询"/>
进阶1 基础查询
(1)下面的语句是否可以执行成功-------yes
SELECT last_name, job_id, salary AS sal
FROM employees;
(2)下面的语句是否可以执行成功------yes
SELECT * FROM employees;
(3)下面的语句是否可以执行成功------yes
注意:as可以省略
SELECT employee_id, last_name,
salary * 12 AS “ANNUAL SALARY”
FROM employees;
(4)显示表departments的结构,并查询其中的全部数据
DESC departments;
SELECT * FROM departments;
(5)显示出表employees中的全部job_id(不能重复)
SELECT DISTINCT job_id FROM employees;
(6)显示出表employees的全部列,各个列之间用逗号连接,列头显示成out_put
注意:① null与任意数拼接都为null
② ifnull(commission_pct, 0)-------将“commission_pct”中为null的数值改为0,其他数值不变
---------------输出1-----------------------
SELECT
IFNULL(commission_pct
, 0),
commission_pct
FROM
employees
;
---------------输出2-----------------------
SELECT
CONCAT(first_name, ‘,’, last_name, ‘,’, job_id, ‘,’, IFNULL(commission_pct, 0)) AS output
FROM
employees
;
---------------输出3-----------------------
SELECT
CONCAT(first_name, ‘,’, last_name, ‘,’, job_id, ‘,’, commission_pct) AS output
FROM
employees
;
(7)isnull(commission_pct), 若数值为null,则返回1。否则返回0。------bool值。1:True,0:False
ifnull(commision_pct,0),若数值为null,则返回0。其他数值不变 ------- 更常用
条件查询:is null 或 is not null 可以判断null值
案例
SELECT ISNULL(commission_pct), commission_pct FROM employees;
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进阶1 基础查询
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