Inside an inner join Select based on input parameter

Introduction

When working with SQL Server, it is common to use stored procedures or queries that accept input parameters. These parameters can be used to filter data in various ways. In this article, we will explore a specific scenario where we need to select data from an inner join based on an input parameter.

Problem Statement

The problem arises when we want to modify the query inside the inner join to include some logic based on the input parameter. However, due to SQL syntax restrictions, placing certain statements within the SELECT clause is not possible.

The Query

The provided query has an IF statement inside the inner join’s SELECT clause, which results in a syntax error:

declare @AreasIds nvarchar(max) = '185,186,194,195,201,202,205,492,483'

select i.Id, i.Title,
a.Areas,
from tblIncidents as i
inner join
(
    IF @AreasIds is not null and @AreaIds != ''
        select i.Id,STRING_AGG(ar.Title,';') as Areas
        from tblIncident i
        left join
        (
            select a.Id, a.Title, ia.Type, ia.GlobalObjectId
            from tblItemArea as ia
            inner join tblArea a on ia.AreaId = a.Id
            where ia.Type = 100 and a.Archived != 'true' and a.IsDeleted != 'true' and a.Id in (SELECT value FROM STRING_SPLIT(@AreaIds, ','))
        ) as ar on ar.GlobalObjectId = i.Id
        group by i.Id
        having STRING_AGG(ar.Title,';') is not null
    ELSE
        select i.Id,STRING_AGG(ar.Title,';') as Areas
        from tblIncident i
        left join
        (
            select a.Id, a.Title, ia.Type, ia.GlobalObjectId
            from tblItemArea as ia
            inner join tblArea a on ia.AreaId = a.Id
            where ia.Type = 100 and a.Archived != 'true' and a.IsDeleted != 'true'
        ) as ar on ar.GlobalObjectId = i.Id
        group by i.Id
) a on i.Id = a.Id
where i.MyCustomFilter = 216 and i.Archived != 'true' and i.IsDeleted != 'true'

Error Message

The error message indicates that there is an incorrect syntax near the keyword IF.

Explanation

In SQL Server, it is not possible to nest conditional statements within a single SELECT clause. This restriction exists due to the way SQL parses queries.

Solution

To solve this problem, we need to rewrite the query to avoid using nested conditional statements. Here are a few approaches:

Approach 1: Using OR and NOT

We can use the OR and NOT operators to achieve similar results without nesting an IF statement:

declare @AreasIds nvarchar(max) = '185,186,194,195,201,202,205,492,483'

select i.Id, i.Title,
a.Areas,
from tblIncidents as i
inner join
(
    select
      i.Id,
      STRING_AGG(ar.Title, ';') as Areas
    from tblIncident i
    left join
    (
        select a.Title,
               ia.GlobalObjectId
        from tblItemArea as ia
        inner join tblArea a on ia.AreaId = a.Id
        where ia.Type = 100
          and a.Archived = 0
          and a.IsDeleted = 0
          and (
              @AreasIds is null or @AreaIds = ''
              or a.Id in (SELECT id FROM @AreaIds)
          )
     ) as ar on ar.GlobalObjectId = i.Id
     group by i.Id
     having @AreasIds is null or @AreaIds = ''
         or STRING_AGG(ar.Title, ';') is not null
) a on i.Id = a.Id
where i.MyCustomFilter = 216
  and i.Archived = 0
  and i.IsDeleted = 0;

This approach uses OR and NOT to evaluate the conditions. However, as we will see later, this query may need further optimization.

Approach 2: Using a Table Variable or Table Value Parameter

A more efficient approach is to use a table variable or Table Value Parameter to store multiple IDs:

declare @AreasIds table (id int PRIMARY KEY);
insert @AreaIds (id) values
(185),
(186),
(194),
(195),
(201),
(202),
(205),
(492),
(483);

declare @withAreaIds bit = 1;

select
  i.Id,
  i.Title, 
  a.Areas,
from tblIncidents as i
inner join
(
    select
      i.Id,
      STRING_AGG(ar.Title, ';') as Areas
    from tblIncident i
    left join
    (
        select
          a.Title,
          ia.GlobalObjectId
        from tblItemArea as ia
        inner join tblArea a on ia.AreaId = a.Id
        where ia.Type = 100
          and a.Archived = 0
          and a.IsDeleted = 0
          and (
              @withAreasIds = 0
              or a.Id in (SELECT id FROM @AreaIds)
              )
     ) as ar on ar.GlobalObjectId = i.Id
     group by i.Id
     having @withAreasIds = 0
            or STRING_AGG(ar.Title, ';') is not null
) a on i.Id = a.Id
where i.MyCustomFilter = 216
  and i.Archived = 0
  and i.IsDeleted = 0;

This approach uses a table variable to store the IDs and then joins it with the main query. This method is more efficient than using nested conditional statements.

Conclusion

When working with SQL Server, it’s essential to understand the syntax restrictions when using input parameters in queries. By using a table variable or Table Value Parameter, we can rewrite the query to achieve similar results without nesting conditional statements. This approach not only resolves the syntax error but also improves the performance of the query.

Last modified on 2024-12-17