Count number of NULLs in a row
Is there a way to get a column indicating the number of NULL fields in a row? This would be within a SELECT statement.
For Example:
Field1 Field2 Num_Null
-----------------------
NULL "A" 1
UPDATE: I want this query so I can sort based on how many Affiliates sales there are of a given Book. So having 3 affiliates would be sorted higher than having 2, regardless of which ones. There are about seven affiliates in my database, and that's subject to grow. So any query requiring that each Affiliate field be specified would probably be too long
The table:
Affiliates_Cache - Primary key is Affiliate_ISBN, has the prices of the boo开发者_StackOverflow中文版k on various affiliates (NULL if its not available). Affiliates_Cache is the one where i want to count the number of NULLs
I'm not sure if there are neater methods, but this should work:
SELECT Field1, Field2, ISNULL(Field1) + ISNULL(Field2) Num_Null
FROM YourTable;
Test case:
CREATE TABLE YourTable (Field1 varchar(10), Field2 varchar(10));
INSERT INTO YourTable VALUES (NULL, 'A');
INSERT INTO YourTable VALUES ('B', 'C');
INSERT INTO YourTable VALUES ('B', NULL);
INSERT INTO YourTable VALUES (NULL, NULL);
Result:
+--------+--------+----------+
| Field1 | Field2 | Num_Null |
+--------+--------+----------+
| NULL | A | 1 |
| B | C | 0 |
| B | NULL | 1 |
| NULL | NULL | 2 |
+--------+--------+----------+
4 rows in set (0.00 sec)
UPDATE: Further to the updated question:
If you have columns in your table that look like affiliate_1
, affiliate_2
, etc, this is rarely a good idea as you would be mixing data with the metadata. In general, a recommended fix is to use another dependent table for the users-to-affiliates relationships, as in the following example:
CREATE TABLE users (
user_id int,
user_name varchar(100),
PRIMARY KEY (user_id)
) ENGINE=INNODB;
CREATE TABLE users_affiliates (
user_id int,
affiliate_name varchar(100),
PRIMARY KEY (user_id, affiliate_name),
FOREIGN KEY (user_id) REFERENCES users (user_id)
) ENGINE=INNODB;
Then sorting the users
table by the number of affiliates will look something like this:
SELECT u.*, d_tb.num_aff
FROM users
JOIN (
SELECT user_id, COUNT(*) num_aff
FROM users_affiliates
GROUP BY user_id
) d_tb ON (d_tb.user_id = u.user_id)
ORDER BY d_tb.num_aff DESC;
The advantages are plenty, but most importantly it makes queries such as the above easy to write, and flexible enough to work with any number of affiliates (an not limited by the number of columns you allocated).
How about this Query ? (Referring to the Test case given by Daniel.)
SELECT Field1, Field2, (2 - (COUNT(ALL Field1)+COUNT(ALL Field2))) Num_Null
FROM @YourTable
GROUP BY Field1, Field2
Keep it simple and Standard:
SELECT Field1, Field2,
CASE WHEN Field1 IS NULL THEN 1 ELSE 0 END
+ CASE WHEN Field2 IS NULL THEN 1 ELSE 0 END
AS Num__Null
FROM YourTable;
Test case in full:
WITH YourTable (Field1, Field2)
AS
(
SELECT CAST(Field1 AS VARCHAR(10)),
CAST(Field2 AS VARCHAR(10))
FROM (
VALUES (NULL, 'A'),
('B', 'C'),
('B', NULL),
(NULL, NULL)
) AS YourTable (Field1, Field2)
)
SELECT Field1, Field2,
CASE WHEN Field1 IS NULL THEN 1 ELSE 0 END
+ CASE WHEN Field2 IS NULL THEN 1 ELSE 0 END
AS Num__Null
FROM YourTable;
精彩评论