MySQL datetime to just date
From the manual here, you can simply use the DATE() function.


 DATE(expr)

Extracts the date part of the date or datetime expression expr.

mysql&gt; SELECT DATE('2003-12-31 01:02:03');
        -&gt; '2003-12-31'



It is designed for precisely your use case. Although the quote is from the 5.7 manual, it works right back to at least 5.0. You don't have to worry about corrupting your data.

If you are using 5.7, you can use a generated column:


  The syntax is:
  
  TYPE   [ GENERATED ALWAYS ]   AS   (  )   [
  VIRTUAL|STORED ] [ UNIQUE [KEY] ]   [ [PRIMARY] KEY ]   [ NOT NULL ]
  [ COMMENT  ]


MyDate DATE AS (DATE(My_Date_Time_Field))  -- or similar, not tested


This avoids throwing away data which might be useful later and nicely tidies your SQL syntax.

However, if you're using an earlier version of MySQL, you can always generate this DATE() part on the fly, which given the relatively simple syntax, shouldn't be too problematic.