SQL query to conditionally select a field value - sql

Question

I have an SQL query that joins 3 tables to return me the required data. The query is as follows:
SELECT (s.user_created,
u.first_name,
u.last_name,
u.email,
s.school_name,
p.plan_name,
substring( a.message, 11), u.phone1)
FROM cb_school s
inner join ugrp_user u
on s.user_created = u.user_id
inner join cb_plan p
on s.current_plan = p.plan_id
inner join audit a
on u.user_id = a.user_id
where s.type = 'sample'
and a.module_short = 'sample-user'
and s.created_time > current_timestamp - interval '10 day';
The query works fine if all the attributes are present. Now for few of my rows, the following value would be a.module_short = 'sample-user' missing. But since I have included it as an AND condition, those rows will not be returned. I am trying to return an empty string for that field if it is present, else the value as per my current query. Is there any way to achieve this.

Answer 1

Think you could possibly use a CASE WHEN statement, like this:
SELECT CASE WHEN a.module_short = 'sample-user' THEN a.module_short
ELSE '' END AS a.module_short
FROM TableA

Answer 2

you can use COALESCE it returns the first not null.
SELECT COALESCE(a.module_short,'')
FROM TableA AS a

Answer 3

SELECT (s.user_created,
u.first_name,
u.last_name,
u.email,
s.school_name,
p.plan_name,
substring( a.message, 11), u.phone1)
FROM cb_school s
INNER JOIN ugrp_user u
ON s.user_created = u.user_id
INNER JOIN cb_plan p
ON s.current_plan = p.plan_id
INNER JOIN audit a
ON u.user_id = a.user_id
AND a.module_short = 'sample-user'
WHERE s.type = 'sample'
AND s.created_time > current_timestamp - interval '10 day';

Answer 4

You want to show all users that have at least one module_short.
If the module_short contains 'sample-user' then it should show it, else it should show NULL as module_short. You only want 1 row per user, even if it has multiple module_shorts.
You can use a CTE, ROW_NUMBER() and the CASE clause for this question.
Example Question
I have 3 tables.
Users: Users with an ID
Modules: Modules with an ID
UserModules: The link between users and modules. You user can have multiple models.
I need a query that returns me all users that have at least 1 module with 2 columns UserName and ModuleName.
I only one 1 row for each user. The ModuleName should only display SQL if the user has that module. Else it should display no module.
Example Tables:
Users:
id name
1 Manuel
2 John
3 Doe
Modules:
id module
1 StackOverflow
2 SQL
3 StackExchange
4 SomethingElse
UserModules:
id module_id user_id
1 1 2
2 1 3
4 2 2
5 2 3
6 3 1
7 3 3
8 4 1
9 4 3
Example Query:
with CTE as (
select
u.name as UserName
, CASE
WHEN m.module = 'SQL' THEN 'SQL' ELSE NULL END as ModuleName
, ROW_NUMBER() OVER(PARTITION BY u.id
ORDER BY (CASE
WHEN m.module = 'SQL' THEN 'Ja' ELSE NULL END) DESC) as rn
from UserModules as um
inner join Users as u
on um.user_id = u.id
inner join Modules as m
on um.module_id = m.id
)
select UserName, ModuleName from CTE
where rn = 1
Example Result:
UserName ModuleName
Manuel NULL
John SQL
Doe SQL
Your query would look like this:
with UsersWithRownumbersBasedOnModule_short as (
SELECT s.user_created,
u.first_name,
u.last_name,
u.email,
s.school_name,
p.plan_name,
substring( a.message, 11),
u.phone1)
CASE
WHEN a.module_short = 'sample-user'
THEN a.module_short
ELSE NULL
END AS ModuleShort
ROW_NUMBER() OVER(PARTITION BY u.user_id ORDER BY (
CASE
WHEN a.module_short = 'sample-user'
THEN a.module_short
ELSE NULL
END) DESC) as rn
FROM cb_school s
inner join ugrp_user u
on s.user_created = u.user_id
inner join cb_plan p
on s.current_plan = p.plan_id
inner join audit a
on u.user_id = a.user_id
where s.type = 'sample'
and s.created_time > current_timestamp - interval '10 day';)
select * from UsersWithRownumbersBasedOnModule_short
where rn = 1
PS: I removed a lose bracket after SELECT and your SUBSTRING() is missing 1 parameter, it needs 3.