MySQL查询在where子句中包含空值

Question

I have a table member1 which looks like this. I am editing my table why I want to return all values from pst.

id  club_id mobile      pst
1     1     9437054788  6
2     1     9437345602  NULL
3     2     9437056738  35
4     2     9437064876  39
5     1     9437059927  NULL
6     2     9437965276  40
7     1     9437121455  NULL

I have a drop-down fields one for club_id and another check-box field for pst with a value of 'select All' on both fields. 'select All' equals '%'.

The solution provided by @TheConstructor works well in this query

SELECT group_concat(mobile) FROM `member1` WHERE COALESCE(pst, '') 
LIKE '%' and club_id=1

or

SELECT group_concat(mobile) FROM `member1` WHERE COALESCE(pst, '') 
LIKE '6' and club_id=1

Now I want to have multiple values in the query in place of '6' for example

SELECT group_concat(mobile) FROM `member1` WHERE COALESCE(pst, '') 
in (6,35,39,40) or in ('%') and club_id=1

How I can achieve that, as at the moment I can return the result with just one value ie 6 or 35 or 39 or 40

Answer 1

As you will have noticed NULL LIKE '%' will not evaluate to TRUE and rows where pst is NULL are excluded from the result. Now there a different approaches to include rows where pst is NULL ; here are three of them:

1. If you want to get all rows you can just leave out the WHERE all together.

    SELECT group_concat(mobile) FROM `member1`; 

2. If you want to always include NULL you can add OR pst IS NULL . (Can use index on column pst )

    SELECT group_concat(mobile) FROM `member1` WHERE pst LIKE '%' OR pst IS NULL; 

   If you plan on changing LIKE '%' for another predicate, this is probably the fastest SQL on bigger tables.

3. If you want to treat NULL as if it was an empty string you can use: (will not use index on column pst )

    SELECT group_concat(mobile) FROM `member1` WHERE COALESCE(pst, '') LIKE '%' 

   COALESCE(pst, '') will give the value inside column pst if it is not NULL or '' if pst is NULL .

   While I regard this to be the most flexible one as you only need to change '%' to something different to start filtering and exclude NULL -values, MySQL's optimizer will sadly handle this differently from the second query which will cost you performance on big tables where pst has an index.

As a last caveat note that GROUP_CONCAT will by default only return 1024 characters. This can be changed by setting group_concat_max_len to a higher value.

Answer 2

Forgive me if I've wildly misunderstood your question, but I think you want just:

SELECT group_concat(mobile) FROM member1

The % in WHERE pst LIKE '%' is a wildcard for zero or more of any character, thus the query will return any NOT NULL pst records.. you are way better off using WHERE pst IS NOT NULL if you need that logic.

If you need to check the actual character % you'll need to escape it; WHERE pst LIKE '\\%' .. but this is the same logic as WHERE pst = '%' as you are checking for equality.

I'm still struggling to understand you but if you are checking for existence in the database of a mobile number the simplest query is:

SELECT 1 FROM member1 WHERE mobile = ?

Where ? , is your pass in variable and any rows returned means the record exists. Make sure you use a binding method that is safe from SQL injection.

I would also shy away from using GROUP_CONCAT in every case except the one when I am displaying the data as CSV. If you are building a drop-down from the results.. just pull in the separate rows in an array and run through it as you build the HTML.

Answer 3

It seems you are using that value as-is, right from the form data. Are you sure this won't result in security issues?

I would grab the value from $_POST, test it, create a variable with the correct WHERE part of the query dependent on the test and use that in the query string.