Informix SQL-简单选择查询返回意外结果

Question

Follow this, it's really obvious and simple. For some reason, the results differ between queries and approach angles on data subsets. Keep in mind that the field, correct_addr is a char(1) field with allowable nulls.

select distinct correct_addr, count(*) from id_rec group by correct_addr;

correct_addr       (count(*))
                         2477
N                          80
Y                       84013

3 row(s) retrieved.

Ok, so correct_addr contains 3 distinct values: "N","Y", and either "" or " " or NULL

So now, I try this:

select count(*) from id_rec where correct_addr <> 'N';

      (count(*))
           84013

What happened to the 2477 records that have that blank value?

Another try from a different angle:

select count(*) from id_rec where correct_addr in (null,'',' ','Y');

      (count(*))
           84013

Same thing happens....

So what is going on here? Why doesn't the sql engine (?) recognize the blank value in the last 2 sql statements? It discovers it fine in the first query with the grouping, but nowhere else.

Does anyone have any ideas as to why this type of thing happens?

Answer 1

NULLs require special handling in SQL.

Try

select count(*) 
from id_rec 
where correct_addr <> 'N' 
    or correct_addr is null; 

See here for an explanation of handling NULLs .

Answer 2

NULL comparisons are always false. And empty string or single space is a value which is not NULL.

However, GROUP BY will recognise it and count it.

Try these

select count(*) from id_rec
where correct_addr <> 'N' or correct_addr IS NULL

select count(*) from id_rec
where COALESCE(correct_addr, 'X') <> 'N' 


select count(*) from id_rec
where COALESCE(correct_addr, ' ') in (' ','Y');

Also, COUNT(column) will ignore NULLS so some more to try

select count(correct_addr), COUNT(*) from id_rec GROUP BY correct_addr

select count(correct_addr), COUNT(*) from id_rec
where correct_addr <> 'N' or correct_addr IS NULL

Note: char(1) will always pad to a space