Question

I’ve a table with a varchar column (A) and another integer column(B) indicating the type of data present in A. If B is 0, then A will always contain numeric digits.

So when I form an sql like this

SELECT COUNT(*) FROM TAB WHERE B = 0 AND TO_NUMBER(A) = 123;

I get an exception invalid number.

I expect B = 0 to be evaluated first, and then TO_NUMBER(A) second, but from the above scenario I suspect TO_NUMBER(A) is evaluated first. Is my guess correct?

Answer 1

In contrast to programming languages like C, C#, Java etc., SQL doesn’t have so called conditional logical operators. For conditional logical operators, the right operand is only evaluated if it can influence the result. So the evaluation of && stops if the left operand returns false. For || it stops if the left operand returns true.

In SQL, both operands are always evaluated. And it’s up to the query optimizer to choose which one is evaluated first.

I propose you create the following function, which is useful in many cases:

FUNCTION IS_NUMBER(P_NUMBER VARCHAR2)
RETURN NUMBER DETERMINISTIC
IS
  X NUMBER;
BEGIN
  X := TO_NUMBER(P_NUMBER);
  RETURN X;
EXCEPTION
  WHEN OTHERS THEN RETURN NULL;
END IS_NUMBER;

Then you can rewrite your query as:

SELECT COUNT(*) FROM TAB WHERE B = 0 AND IS_NUMBER(A) = 123;

You can also use the function to check whether a string is a number.