Question

I have a large table with a DATETIME column and for index reasons I want to add a column which just contains a DATE type. It seems that MySQL is not able to use an index by the following expression GROUP BY DATE(datetime) therefore I want to add another column with a second index.

For updating I use this simple statement:

UPDATE table SET datecol = DATE(datetimecol)

Now a strange behavior occurs: the datecol-column contains the correct values. But the datetimecolumn changes as well: to the current timestamp. This is the default value for this column.

I’m working now for many years with databases and MySQL but I cannot explain this behavior.

The current version is MySQL 5.1.66-0.

Do you have any suggestions or explanations for this?

Answer 1

Your datetimecol is not of type DATETIME, but of type TIMESTAMP which (by default) has the ON UPDATE CURRENT_TIMESTAMP attribute that automatically updates to the current time when a record is modified.

To suppress automatic properties for the first TIMESTAMP column, do either of the following:

• Define the column with a DEFAULT clause that specifies a constant default value.

• Specify the NULL attribute. This also causes the column to permit NULL values, which means that you cannot assign the current timestamp by setting the column to NULL. Assigning NULL sets the column to NULL.

Therefore:

ALTER TABLE my_table
  MODIFY datetimecol TIMESTAMP DEFAULT CURRENT_TIMESTAMP;

To update the records this one time without altering the values in datetimecol, you should explicitly set them to their incumbent values:

UPDATE my_table SET
  datetimecol = datetimecol,
  datecol     = DATE(datetimecol);