Tables :: Empty Fields In Some Records

In my table, I got text fields and numeric fields, however, sometimes, user put an empty field in some records which I disliked. I try to put some checking such as following syntax to check those empty field but nothing happened.

1) If len(NAME) = 0 THEN error message : for TEXT field checking
2) IF len(name) = NULL then error MESSAGE : for TEXT field CHECKING
3) IF NAME = "" then error message : for text field checking
4) IF LEN(AMT) = 0 THEN error message : FOR NUMERIC field checking
5) IF LEN(AMT) = NULL then ERROR MESSAGE : for numeric field checking
6) IF AMT = "" THEN error message : for numeric field checking
7) IF AMT = NULL then ERROR message : for numeric field checking