如何在同一个表中查找具有 null 而不是 null 值的列

Question

I have a table where I need to find a column with null and not null values. for eg The column has a value today but becomes null tomorrow.

First Query:

select * from test where Account Number is Not Null and LastChangedDate is Null  

But this query brings the records with null values and not listing the records with values for LastChangedDate column

Below is the sample record

AccountNumber EffectiveDate LastChangedDate 
1234567     2019-05-31          2018-09-14
1234567     2019-06-30          2018-09-14
1234567     2019-07-31           NULL

Second query:

select * from test where Account Number is Not Null and 'LastChangedDate' is Null  

But this query brings the records with null values and not listing the records with values for 'LastChangedDate' column

Expected: Able to retrieve all records for same column with Null and Not Null values and omit the records with all values populated

Answer 1

Use LEAD() :

SELECT *
FROM (
    SELECT 
        t.*, 
        LEAD(LastChangedDate) OVER(PARTITION BY AccountNumber ORDER BY EffectiveDate) LastChangedDateLead
    FROM test t
) x
WHERE LastChangedDate IS NULL OR LastChangedDateLead IS NULL

This will give you the records where LastChangedDate is NULL or for which the next record (for the same customer and ordered by EffectiveDate ) has a NULL value as LastChangedDate .

NB: the latest record of each customer will show up in the result: since there is no next record, the query will consider that its LastChangedDate is NULL too.

This demo on DB fiddle with your sample data returns:

AccountNumber | EffectiveDate       | LastChangedDate     | LastChangedDateLead
------------: | :------------------ | :------------------ | :------------------
      1234567 | 30/06/2019 00:00:00 | 14/09/2018 00:00:00 | null               
      1234567 | 31/07/2019 00:00:00 | null                | null               

Answer 2

Just do it together:

SELECT *
FROM test
WHERE (Account Number is Not Null and 'LastChangedDate' is Null)