如何在SQL中将字符串日期转换为日期时间类型

Question

I am having old database and in those tables every entry maintains create_at abd updated_at entries,

But this entries are in varchar format eg: 24/04/2018 14:25:29 pm

How to I convert this to datetime so I can fire query and get count of records of each month.

Query will be something like this:

SELECT *
FROM table
WHERE MONTH(columnName) = MONTH(CURRENT_DATE())
AND YEAR(columnName) = YEAR(CURRENT_DATE())   

Answer 1

MySQL:

Use STR_TO_DATE function to convert into datetime

Like this:

SELECT *
FROM table
WHERE MONTH(STR_TO_DATE(columnName,'%d%m%Y')) = MONTH(STR_TO_DATE(columnName,'%d%m%Y'))
AND YEAR(STR_TO_DATE(columnName,'%d%m%Y')) = YEAR(STR_TO_DATE(columnName,'%d%m%Y')) 

Demo:

http://sqlfiddle.com/#!9/94c4b/26

Explanation:

STR_TO_DATE

Syntax: STR_TO_DATE(str,format)

It takes a string str and a format string format.
STR_TO_DATE() returns a DATETIME value if the format string contains both date and time parts, or a DATE or TIME value if the string contains only date or time parts.
If the date, time, or datetime value extracted from str is illegal, STR_TO_DATE() returns NULL and produces a warning.

Specifiers in the format:

%d - Day of the month, numeric (00..31)
%m - Month, numeric (00..12)
%Y - Year, numeric, four digits
%T - Time, 24-hour (hh:mm:ss)

Links To Source:

https://dev.mysql.com/doc/refman/8.0/en/date-and-time-functions.html#function_str-to-date

https://dev.mysql.com/doc/refman/8.0/en/date-and-time-functions.html#function_date-format

Answer 2

Just use substr() or substring() (depending on your database):

SELECT *
FROM table
WHERE substring(columnName, 4, 2) = MONTH(CURRENT_DATE()) AND
      substring(columnName, 7, 4) = YEAR(columnName);

I would then strongly advise you to change the format of the date/time. The exact logic depends on the database, but in general, you can do:

update table
    set columnName = <convert columnname to date/time>;  -- converts to a date and then back to a string using "default" format

alter table <table> alter columnName datetime;  -- "datetime" is probably the appropriate type, but that depends on the database