mysql学习笔记（2）

日期函数

返回当前的日期 curdate()(注意只有一个r),current_date()

SELECT CURDATE(),CuRRENT_DATE() FROM DUAL;

运行结果：

+------------+----------------+

| CURDATE() | CuRRENT_DATE() |

+------------+----------------+

| 2018-10-12 | 2018-10-12 |

+------------+----------------+

返回当前的时间 curtime(),current_time()

SELECT CURTIME(),CURRENT_TIME() FROM DUAL;

运行结果：

+-----------+----------------+

| CURTIME() | CURRENT_TIME() |

+-----------+----------------+

| 11:18:49 | 11:18:49 |

+-----------+----------------+

返回当前的日期和时间 now(),current_timestamp(),sysdate(),localtime(),localtimestamp()

SELECT NOW(),CURRENT_TIMESTAMP(),SYSDATE(),LOCALTIME(),LOCALTIMESTAMP() FROM DUAL;

运行结果：

+---------------------+---------------------+---------------------+---------------------+---------------------+

| NOW() | CURRENT_TIMESTAMP() | SYSDATE() | LOCALTIME() | LOCALTIMESTAMP() |

+---------------------+---------------------+---------------------+---------------------+---------------------+

| 2018-10-12 11:19:15 | 2018-10-12 11:19:15 | 2018-10-12 11:19:15 | 2018-10-12 11:19:15 | 2018-10-12 11:19:15 |

+---------------------+---------------------+---------------------+---------------------+---------------------+

返回日期date中的年 YEAR(date)

返回日期date中的月 month(date) 返回阿拉伯数字； monthname(date) 返回英文月份

SELECT YEAR('2018-09-03'),MONTH('2018-10-9'),MONTHNAME('2018-10-9') FROM DUAL;

运行结果：

+--------------------+--------------------+------------------------+

| YEAR('2018-09-03') | MONTH('2018-10-9') | MONTHNAME('2018-10-9') |

+--------------------+--------------------+------------------------+

| 2018 | 10 | October |

+--------------------+--------------------+------------------------+

SELECT YEAR(now()),YEAR(CURTIME()) FROM DUAL;

运行结果：

+-------------+-----------------+

| YEAR(now()) | YEAR(CURTIME()) |

+-------------+-----------------+

| 2018 | 2018 |

+-------------+-----------------+

返回日期date中月份的第几天 dayofmonth(date)

SELECT DAYOFMONTH(CURTIME()) FROM DUAL;

运行结果：

+-----------------------+

| DAYOFMONTH(CURTIME()) |

+-----------------------+

| 12 |

+-----------------------+

返回日期date中年的第几天 dayofyear(date)

SELECT DAYOFYEAR(NOW()),DAYOFYEAR('2008-12-30'),DAYOFYEAR('2018-12-30') FROM DUAL;

运行结果：

+------------------+-------------------------+-------------------------+

| DAYOFYEAR(NOW()) | DAYOFYEAR('2008-12-30') | DAYOFYEAR('2018-12-30') |

+------------------+-------------------------+-------------------------+

| 285 | 365 | 364 |

+------------------+-------------------------+-------------------------+

hour(time) 返回时间time中的时
minute(time) 返回时间time中的分

second(time) 返回时间time中的秒

SELECT CURTIME(), HOUR(CURTIME()),MINUTE(NOW()),SECOND(NOW()) FROM DUAL;

运行结果：

+-----------+-----------------+---------------+---------------+

| CURTIME() | HOUR(CURTIME()) | MINUTE(NOW()) | SECOND(NOW()) |

+-----------+-----------------+---------------+---------------+

| 11:24:28 | 11 | 24 | 28 |

+-----------+-----------------+---------------+---------------+

extract(type) 从日期date中返回指定type的值
- SELECT
  
      EXTRACT(YEAR FROM NOW()) 年,EXTRACT(MONTH FROM NOW()) 月,
  
      EXTRACT(DAY FROM NOW()) 日,EXTRACT(HOUR FROM NOW()) 时,
  
      EXTRACT(MINUTE FROM NOW()) 分,EXTRACT(SECOND FROM NOW()) 秒
  
  FROM DUAL;

运行结果：
+------+------+------+------+------+------+
| 年   | 月   | 日   | 时   | 分   | 秒   |
+------+------+------+------+------+------+
| 2018 |   10 |   12 |   11 |   30 |   59 |
+------+------+------+------+------+------+

adddate(date,n) 返回前面日期加上多少天；可用于计算产假，入职多少天等

SELECT ADDDATE(NOW(),50);

运行结果：

+---------------------+

| ADDDATE(NOW(),50) |

+---------------------+

| 2018-12-01 11:31:27 |

+---------------------+

addtime(date,n) 返回前面日期加上多少秒，n最大59

SELECT NOW(),ADDTIME(NOW(),59),ADDTIME(NOW(),60) FROM DUAL;

运行结果：

+---------------------+---------------------+-------------------+

| NOW() | ADDTIME(NOW(),59) | ADDTIME(NOW(),60) |

+---------------------+---------------------+-------------------+

| 2018-10-12 11:31:52 | 2018-10-12 11:32:51 | NULL |

+---------------------+---------------------+-------------------+

datediff(date1,date2) 返回 date1 - date2 的值，有绝对值

SELECT DATEDIFF(NOW(),'1993-05-07'),DATEDIFF('1993-05-07',NOW()) FROM DUAL; #计算从出生到现在有多少天

运行结果：

+------------------------------+------------------------------+

| DATEDIFF(NOW(),'1993-05-07') | DATEDIFF('1993-05-07',NOW()) |

+------------------------------+------------------------------+

| 9289 | -9289 |

+------------------------------+------------------------------+

格式化日期 date_format(date,str) str 有【 %Y%M%D】、【 %y%m%d 】、【 %y/%m/%d 】、【 %Y/%M/%D 】

SELECT DATE_FORMAT('2018-08-11','%Y%M%D') FROM DUAL;

运行结果：

+------------------------------------+

| DATE_FORMAT('2018-08-11','%Y%M%D') |

+------------------------------------+

| 2018August11th |

+------------------------------------+

SELECT DATE_FORMAT('2018-08-11','%y%m%d') FROM DUAL;

运行结果：

+------------------------------------+

| DATE_FORMAT('2018-08-11','%y%m%d') |

+------------------------------------+

| 180811 |

+------------------------------------+

SELECT DATE_FORMAT('2018-08-11','%Y/%M/%D') FROM DUAL;

运行结果：

+--------------------------------------+

| DATE_FORMAT('2018-08-11','%Y/%M/%D') |

+--------------------------------------+

| 2018/August/11th |

+--------------------------------------+

SELECT DATE_FORMAT('2018-08-11','%y/%m/%d') FROM DUAL;

运行结果：

+--------------------------------------+

| DATE_FORMAT('2018-08-11','%y/%m/%d') |

+--------------------------------------+

| 18/08/11 |

+--------------------------------------+

SELECT DATE_FROMAT('2018-08-11','%y/%m/%d') FROM DUAL;

代码：

+--------------------------------------+

| DATE_FORMAT('2018-08-11','%y/%m/%d') |

+--------------------------------------+

| 18/08/11 |

+--------------------------------------+

判断空否 ifnull(v1,v2) 如果v1值不为null，则返回v1，否则返回v2

SELECT vend_state,IFNULL(VEND_STATE,'SZ') AS 城市 FROM vendors;

运行结果：

+------------+--------+

| vend_state | 城市 |

+------------+--------+

| MI | MI |

| OH | OH |

| CA | CA |

| NY | NY |

| NULL | SZ |

| NULL | SZ |

+------------+--------+

if(exp,v1,v2) 如果表达式exp成立，返回v1,否则返回v2

SELECT vend_state,IF(VEND_STATE IS NULL,'SZ',VEND_STATE) AS 城市 FROM vendors; #(等同与上者)

运行结果：

+------------+--------+

| vend_state | 城市 |

+------------+--------+

| MI | MI |

| OH | OH |

| CA | CA |

| NY | NY |

| NULL | SZ |

| NULL | SZ |

+------------+--------+

SELECT prod_name,prod_price,IF(prod_price > 5,'贵','便宜') AS 评价 FROM products;

运行结果：

+----------------+------------+--------+

| prod_name | prod_price | 评价 |

+----------------+------------+--------+

| .5 ton anvil | 5.99 | 贵 |

| 1 ton anvil | 9.99 | 贵 |

| 2 ton anvil | 14.99 | 贵 |

| Detonator | 13.00 | 贵 |

| Bird seed | 10.00 | 贵 |

| Carrots | 2.50 | 便宜 |

| Fuses | 3.42 | 便宜 |

| JetPack 1000 | 35.00 | 贵 |

| TNT (5 sticks) | 10.00 | 贵 |

+----------------+------------+--------+

多重判断 case when exp1 than '别名' [when exp2 than '别名'] [...] end 列别名

代码：

SELECT vend_id,prod_name,prod_price,

case

when prod_price <= 10 then '便宜'

when prod_price>10 && prod_price <= 20 then '比较贵'

when prod_price>20 then '贵'

end 评价

FROM products;

运行结果： +---------+----------------+------------+-----------+ | vend_id | prod_name | prod_price | 评价 | +---------+----------------+------------+-----------+ | 1001 | .5 ton anvil | 5.99 | 便宜 | | 1001 | 1 ton anvil | 9.99 | 便宜 | | 1001 | 2 ton anvil | 14.99 | 比较贵 | | 1003 | Detonator | 13.00 | 比较贵 | | 1003 | Bird seed | 10.00 | 便宜 | | 1005 | JetPack 1000 | 35.00 | 贵 | | 1005 | JetPack 2000 | 55.00 | 贵 | | 1002 | Oil can | 8.99 | 便宜 | | 1003 | Safe | 50.00 | 贵 | +---------+----------------+------------+-----------+

系统信息函数

返回数据库的版本号： version()
返回服务器的链接数： connection_id()
返回当前的数据库名： database(),schema()
返回当前用户： user(),system_user(),current_user()

加密函数： password(str),md5(str)

运行结果：

+-------------------------------------------+----------------------------------+

| password('root') | md5('root') |

+-------------------------------------------+----------------------------------+

| *81F5E21E35407D884A6CD4A731AEBFB6AF209E1B | 63a9f0ea7bb98050796b649e85481845 |

+-------------------------------------------+----------------------------------+

格式化函数

将数字进行格式格式化：format(x,n) （n数字，n截取的小数位）

SELECT FORMAT(1234.1234,2) FROM DUAL;

运行结果：

+---------------------+

| FORMAT(1234.1234,2) |

+---------------------+

| 1,234.12 |

+---------------------+

ascll(str) 返回字符串第一个字符的ASCII值
bin(str) 十进制转二进制

数值函数
最大值 max() 空值不会参与计算
最小值 min() 空值不会参与计算
总计 sum() 空值不会参与计算

平均数 avg() 空值不会参与计算

SELECT MAX(prod_price)最大值,min(prod_price) 最小值,AVG(prod_price) 总计,AVG(prod_price)平均数 FROM products;

运行结果：

+-----------+-----------+-----------+-----------+

| 最大值 | 最小值 | 总计 | 平均数 |

+-----------+-----------+-----------+-----------+

| 55.00 | 2.50 | 16.133571 | 16.133571 |

+-----------+-----------+-----------+-----------+

计数 count()

count(*) 会统计非空； count(column) 空值不会参与计算

SELECT COUNT(),COUNT(VEND_STATE) FROM VENDORS;

运行结果：

+----------+-------------------+

| COUNT() | COUNT(VEND_STATE) |

+----------+-------------------+

| 6 | 4 |

+----------+-------------------+