UNION SELECT MySQL的PHP​​的PDO沒有價值迴歸

Question

SELECT t1.case_num AS casenum, NULL, NULL, NULL, NULL, NULL, NULL, NULL, 
    NULL, NULL 
FROM luponcase_tbl t1 
WHERE t1.case_num = :id AND t1.status = :active 
UNION 
SELECT mhearing_number, mhearing_status, mhearing_number, NULL, NULL, NULL, NULL, NULL, 
    NULL, NULL 
FROM luponcase_mediation_tbl t4 
WHERE t4.case_num = :id AND t4.status = :active 
UNION 
SELECT mfileblob, mfile_type, mfilesize, mhearing_number, mnameoffile, NULL, NULL, NULL, 
    NULL, NULL 
FROM luponcase_mediation_reference_tbl t5 
WHERE t5.case_num = :id AND t5.status = :active 

這是我的工會SQL和我沒有得到結果，於是，我就

while ($selected_row = $stmt->fetch(PDO::FETCH_ASSOC)) { 
         print_r($selected_row); 
} 

迴應令我驚訝的結果，結果是

Array ([casenum] => 14 [NULL] =>) 

我假設第一個值是好的，然後NULL列不是。如何使它成爲我只會得到除NULL之外的列的方式。

Answer 1

當您連接到數據庫，你可以設置一些屬性來控制PDO如何處理空值和空字符串時，他們是由數據庫查詢