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CREATE DATABASE TestDB;
GO
USE TestDB;
CREATE TABLE test (
id int IDENTITY,
pdate datetime);
INSERT test
VALUES('1-Feb-2018'),('1-Mar-2018'),(NULL);

SELECT * FROM test

SELECT * FROM
    (
        SELECT
            id,
            IIF(pdate IS NULL, '', FORMAT(pdate, 'yyyy-MM-dd')) pdate2
        FROM test
    ) a
WHERE pdate2 = ''

SELECT * FROM
    (
        SELECT
            id,
            IIF(pdate IS NULL, '', FORMAT(pdate, 'yyyy-MM-dd')) pdate2
        FROM test
    ) a
WHERE pdate2 = '    '

SELECT * FROM
    (
        SELECT
            id,
            IIF(pdate IS NULL, '', FORMAT(pdate, 'yyyy-MM-dd')) pdate2
        FROM test
    ) a
WHERE pdate2 LIKE '    '