SQL中爲NULL的情況

Question

我有這段代碼，但它不起作用。

當NULL存在時，它應該給出'沒有標題但沒有標題。

SELECT employee.pin, 
CASE gender 
WHEN 'M' or (employee.titel IS NULL) THEN CONCAT('Mister ', ' ', ' ', person.fname, ', ') 
WHEN 'W' or (employee.titel IS NULL) THEN CONCAT('Mrs ', ' ', ' ',  person.fname, ', ') 
WHEN 'M' THEN CONCAT('Mister ', employee.titel, ' ', person.fname, ', ') 
WHEN 'W' THEN CONCAT('Mrs ', employee.titel, ' ', person.fname, ', ') 
END AS salutation 
FROM person,employee 

where person.pin=employee.pin 
ORDER BY pin 

Answer 1

您可以使用CONCAT_WS將直接跳過空值：

SELECT 
    employee.pin, 
    CASE WHEN gender='M' THEN CONCAT_WS(' ', 'Mister', employee.titel, person.fname) 
     WHEN gender='W' THEN CONCAT_WS(' ', 'Mrs', employee.titel, person.fname) 
    END AS salutation 
FROM 
    person INNER JOIN employee 
    ON person.pin=employee.pin 
ORDER BY pin 

Answer 2

你將不得不使用其他CASE語法，因爲NULL不等於什麼：

CASE 
WHEN gender='M' or (employee.titel IS NULL) THEN CONCAT('Mister ', ' ', ' ', person.fname, ', ') 
WHEN gender='W' or (employee.titel IS NULL) THEN CONCAT('Mrs ', ' ', ' ',  person.fname, ', ') 
WHEN gender='M' THEN CONCAT('Mister ', employee.titel, ' ', person.fname, ', ') 
WHEN gender='W' THEN CONCAT('Mrs ', employee.titel, ' ', person.fname, ', ') 
WHEN gender IS NULL THEN ... 
END AS salutation