Using distinct

We will now discuss another kind of query: the distinct query. Firstly, however, we need to introduce another very useful function for the DBA called the coalesce function. The coalesce function, given two or more parameters, returns the first value that is not NULL.

For example, let’s use the coalesce function for the test value:

forumdb=> select coalesce(NULL,'test');
 coalesce
----------
 test
(1 row)

In the preceding query, the coalesce function returns test because the first argument is NULL and the second argument is not NULL.

Now, let’s insert a new category:

forumdb=> insert into categories (title) values ('New Category');
INSERT 0 1

And then let’s perform the following query:

forumdb=# \pset null (NULL)
Null display is "(NULL)".
forumdb=> select pk,title,description from categories;
 pk |         title         |           description      
----+-----------------------...