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SELECT COUNT in Database

This is a common problem often overlooked by Java programmer.

Create the following table in your database

   1: create table test_table
   2: ( 
   3: field_1 varchar2(10) not null,
   4: field_2 varchar2(10)
   5: );
   6:  
   7:  
   8: insert into test_table values('1','1');
   9:  
  10: insert into test_table values('2', null);

Do

select count(*) from test_table

The number of records = 2

Do

select count(field_1) from test_table

The number of records = 2

Do

select count(field_2) from test_table

The number of records = 1. This is because it has NULL value in the 2nd record

Same for

select count(distinct field_2) from test_table

NULL value is excluded.


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