SQL进阶-数据类型转换

一般有这几种转换：

一、数值型与字符型的转化

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数值型转为字符型：
    65 –> '65'               select cast(65 as char);
    65 –> 'A'                select char(65);
    65.23 --> '65.23'        select cast(65.23 as char(10));


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字符型转为数值型：
    '65.23' –> 65            select cast('65.23' as signed int);
    '65.23' --> 65.23        select cast('65.23' as decimal(5,2));
    'A' –> 65                select ASCII('A');

二、数值型与日期和时间型的转化

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数值型转为日期和时间型：
        20190903 --> 日期2019-09-03                                select cast(20190903 as date);

        815 --> 时间00:08:15                                       select cast(815 as time);

        0200815002356 --> 时间戳2020-08-15 00:23:56                select cast(20200815002356 as datetime);
    
        ##把Unix时间戳转为日期
        1628434346325 --> 2021-08-08 22:52:26                     select from_unixtime(1628434346325/1000, '%Y-%m-%d %H:%i:%s');
    

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日期时间型转为数值型：
    如：2019-09-03 –> 20190903                        select cast(current_date() as signed);

    如：14:04:10 –> 140410                            select cast(current_time() as signed);

    如：2019-09-03 14:04:10 --> 20190903140410        select cast(now() as signed);

    如：如：2019-09-03 –> 2019                         select year(current_date());

    如：14:04:10 –> 10                                select second(current_time);

    ##把日期类型转为Unix时间戳
    select unix_timestamp('2019-09-03 11:47:30');

三、字符型与日期和时间型的转化

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字符型转换为日期时间型：
    ‘20190903’ --> 日期2019-09-03              select cast('20190903' as date);

    '2019-09-03' --> 日期2019-09-03            select cast('2019-09-03' as date);

    ‘2109/09/03’ --> 日期2019-09-03            select cast('2019/09/03' as date);

    ‘2019#09#03’ --> 日期2019-09-03            select cast('2019#09#03' as date);

    '00:08:15' --> 时间00:08:15                select cast('00:08:15' as time);

    '20210808225226' --> 时间戳2020-08-15 00:08:15         select CAST('20210808225226' AS DATETIME);
    
    '2021-08-08 22:52:26' --> 时间戳2021-08-08 22:52:26    select cast('2021-08-08 22:52:26' as datetime);
    


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日期时间型转为字符型：
    如：日期2019-09-03 –> '20190903'                select DATE_FORMAT(CURRENT_DATE(),'%Y%m%d')

    如：日期2019-09-03 --> '2019-09-03'             select CAST(CURRENT_DATE() AS DATE);
                                                  select DATE_FORMAT(CURRENT_DATE(),'%Y-%m-%d');

    如：日期2020-08-15' --> '2020-08'              select DATE_FORMAT(CURRENT_DATE(),'%Y-%m');

    时间00:08:15 --> '00:08:15'                    select DATE_FORMAT(NOW(),'%H:%i:%s');
                                                  select CAST(CURRENT_TIME() AS CHAR);

    时间戳2020-08-15 00:08:15 –> '20210808225226'          select DATE_FORMAT(NOW(),'%Y%m%d%H%i%s')

    时间戳2021-08-08 22:52:26 --> '2021-08-08 22:52:26'    select CAST(NOW() AS CHAR);
                                                          select DATE_FORMAT(NOW(),'%Y-%m-%d %H:%i:%s');

    日期2020-08-15 --> Thursday                select DAYNAME(CURRENT_DATE());

    日期2020-08-15 --> August                select MONTHNAME(CURRENT_DATE());