How do I check if a column is empty or null in MySQL?

A shorter way to write the condition:

WHERE some_col > ''

Since null > '' produces unknown, this has the effect of filtering out both null and empty strings.

As defined by the SQL-92 Standard, when comparing two strings of differing widths, the narrower value is right-padded with spaces to make it is same width as the wider value. Therefore, all string values that consist entirely of spaces (including zero spaces) will be deemed to be equal e.g.

'' = ' ' IS TRUE
'' = '  ' IS TRUE
' ' = '  ' IS TRUE
'  ' = '      ' IS TRUE
etc

Therefore, this should work regardless of how many spaces make up the some_col value:

SELECT * 
  FROM T
 WHERE some_col IS NULL 
       OR some_col = ' ';

or more succinctly:

SELECT * 
  FROM T
 WHERE NULLIF(some_col, ' ') IS NULL;

This will select all rows where some_col is NULL or '' (empty string)

SELECT * FROM table WHERE some_col IS NULL OR some_col = '';