Question

I have some database where the previous err…person has put the date format in as 20121218, is it possible to use this as a date in mysql so

SELECT substr(week_begin_date,0,4)+'-'+substr(week_begin_date,4,2)+'-'+substr(week_begin_date,6,8) 
AS startdate 
FROM table 
WHERE startdate < now() 

Answer 1

As stated in Date and Time Literals:

MySQL recognizes DATE values in these formats:

• As a string in either 'YYYY-MM-DD' or 'YY-MM-DD' format. A “relaxed” syntax is permitted: Any punctuation character may be used as the delimiter between date parts. For example, '2012-12-31', '2012/12/31', '2012^12^31', and '2012@12@31' are equivalent.

• As a string with no delimiters in either 'YYYYMMDD' or 'YYMMDD' format, provided that the string makes sense as a date. For example, '20070523' and '070523' are interpreted as '2007-05-23', but '071332' is illegal (it has nonsensical month and day parts) and becomes '0000-00-00'.

• As a number in either YYYYMMDD or YYMMDD format, provided that the number makes sense as a date. For example, 19830905 and 830905 are interpreted as '1983-09-05'.

Therefore 20121218 is actually a valid date literal. Therefore:

SELECT DATE(`week_begin_date`) AS `startdate`
FROM   `table`
WHERE  DATE(`week_begin_date`) < NOW()

See it on sqlfiddle.

However, for the longer-term, you would be wise to fix your schema (and, obviously, any application code that expects to read values in the existing format):

ALTER TABLE `table` MODIFY week_begin_date DATE

See it on sqlfiddle.