Convert Date and Filter by Year

Question

I have a date field that is in a 'yyyymmdd' format, and I need to filter the query by the year, using that field.

For example, in my table there is a "Posting Date" field that is in the '20180401' format, and I need to filter the query to only the records with a year of 2018.

Answer 1

It would be much better if you used a proper type for dates, i.e. DATE and not VARCHAR, as that would give you several advantages:

• integrity: prevents nonsense data to be inserted in the column, like `20181301', '20190133', '2018ABCd', 'today', 'Xmas19', etc.
• storage and index size: DATE is 3 bytes vs 9 of VARCHAR(8).
• usability: let you use the ready made conversions and format functions (not for the specific query but possibly for other ones).

If you can't change the type, it's not so bad though as your format is at least an order preserving one, so you can use conditions that can utilize indexes:

WHERE PostingDate >= '20180101' 
  AND PostingDate  < '20190101'

which by the way can still be used when (and if) you convert the column type to DATE. Nothing would need to change.