对CASE Mysql进行空检查

Question

I have tried to check value, and if is it null(nothing to show) I deduce empty string. I was trying :

SELECT nullif(order_id, ' ') as order_id FROM label_pass_voucher WHERE order_id = 3447;
select CASE WHEN order_id then order_id else ' ' end as order_id FROM label_pass_voucher WHERE order_id = 3447;

it doesn't deduce nothing. Where is my fault? Thanks in advance

Answer 1

Use IFNULL(exp1, exp2)

SELECT IFNULL(order_id, ' ') FROM label_pass_voucher WHERE order_id = 3447;

This syntax equivalent to exp1 == null ? exp2 : exp1 exp1 == null ? exp2 : exp1

Answer 2

I assume that the query you're looking for is:

SELECT IFNULL(order_id, '') AS order_id
FROM label_pass_voucher
WHERE order_id = 3447;

MySQL official documentation:

If expr1 is not NULL, IFNULL() returns expr1; otherwise it returns expr2.

NULLIF() does a slightly different thing:

Returns NULL if expr1 = expr2 is true, otherwise returns expr1. This is the same as CASE WHEN expr1 = expr2 THEN NULL ELSE expr1 END.