MySQL - Change date string to date type in place?

Question

(Hi all. I'm new-ish to SQL and MySQL in particular. I know some PHP.)

I have a table which contains a "date_string" column. For legacy reasons, this is a text field, containing the date in d/m/YY format. e.g. "22/11/09" for 22nd November 2009.

Question: how could I convert all fields in this column to a standard MySQL date format (YYYY-mm-dd), in-place?

Or, failing being able to change it in place, given that the column type is text, how could I take each existing text-based date, and create a standard date in another column (of type 'date') within the table?

Thanks for any help.

Solution 1

You probably want to use the STR_TO_DATE() function.

SELECT STR_TO_DATE(textdate, '%d/%m/%y') FROM MyTable...

Or create another column with DATE data type and copy the values:

ALTER TABLE MyTable ADD COLUMN realdate DATE;
UPDATE MyTable SET realdate = STR_TO_DATE(textdate, '%d/%m/%y');

Solution 2

    UPDATE your_table
    SET your_col = '20'
              + substring(your_col, 
                          LOCATE('/', your_col, LOCATE('/', your_col) + 1) + 1
                         )
              + '-'
              + substring(your_col, 
                          LOCATE('/', your_col) + 1, 
                          LOCATE('/', your_col, LOCATE('/', your_col) + 1)
                         )
              + '-'
              + substring(your_col, 1, LOCATE('/', your_col) - 1)


/*
    LOCATE('/', your_col) -> search for 1st occurence of '/' in your_col
    LOCATE('/', your_col, LOCATE('/', your_col) + 1) -> search for 2nd occurence of '/' in your_col
    LOCATE('/', your_col, LOCATE('/', your_col) + 1) + 1 -> from where to retriev the last part
*/

(I suppose year >= 2000)

Comments

[SirRatty]

Thank you for the help! When I try either option, I get "NULL" as the results for all fields. Specifically, if I do this: SELECT STR_TO_DATE('%d/%m/%y', date_string) FROM myTable Then I get a list of NULL values, one for each record. "date_string" is of varchar type, 32 chars max. Dates are currently stored as "19/3/05", "1/8/07", etc.

[SirRatty]

Thank you. Yes, the year in this case is always >= 2000. However, the strings themselves are of variable length, as "1/1/01" has been recorded along with dates like "31/12/09". Would both dates survive your algorithm? Cheers.

[Bill Karwin]

Sorry, I got the arguments reversed. I'll edit the above examples to be correct.

[manji]

1st version will not apply to '1/1/01'. Amended. Now it will work for both.