Check out example codes for "which takes more space in a database a datetime or separate date and time columns?". It will help you in understanding the concepts better.

Code Example 1

/* If you have need of both a date and time, it is usually better to use a 
   datetime datatype. It makes most queries slightly easier by not having
   to combine data from different columns to get a particular moment.
   It visually takes up less space in a DB, and it is also more common 
   practice, so has wider support.
   Exceptions include when legacy support is needed for separate fields.   */

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