Using an equality check between columns in a SELECT clause - sql

Question

I am using Microsoft SQL Server 2012 and I would like to run this seemingly simple query:
SELECT
FirstEvent.id AS firstEventID,
SecondEvent.id AS secondEventID,
DATEDIFF(second, FirstEvent.WndFGEnd, SecondEvent.WndFGStart) AS gap,
FirstEvent.TitleID = SecondEvent.TitleID AS titlesSameCheck
FROM VibeFGEvents AS FirstEvent
RIGHT OUTER JOIN VibeFGEvents AS SecondEvent
ON
FirstEvent.intervalMode = SecondEvent.intervalMode
AND FirstEvent.id = SecondEvent.id - 1
AND FirstEvent.logID = SecondEvent.logID
However FirstEvent.TitleID = SecondEvent.TitleID AS titlesSameCheck in the SELECT clause is incorrect syntax. But the SELECT Clause (Transact-SQL) documentation includes this syntax:
SELECT [ ALL | DISTINCT ]
[ TOP ( expression ) [ PERCENT ] [ WITH TIES ] ]
<select_list>
<select_list> ::=
{
*
| { table_name | view_name | table_alias }.*
| {
[ { table_name | view_name | table_alias }. ]
{ column_name | $IDENTITY | $ROWGUID }
| udt_column_name [ { . | :: } { { property_name | field_name }
| method_name ( argument [ ,...n] ) } ]
| expression
[ [ AS ] column_alias ]
}
| column_alias = expression
} [ ,...n ]
I think that means an expression is valid in the select clause and indeed the examples given include things like 1 + 2. Looking at the documentation for expressions:
{ constant | scalar_function | [ table_name. ] column | variable
| ( expression ) | ( scalar_subquery )
| { unary_operator } expression
| expression { binary_operator } expression
| ranking_windowed_function | aggregate_windowed_function
}
boolean equality checks are valid expressions and indeed the example expression given in the = (Equals) (Transact-SQL) documentation includes one:
SELECT DepartmentID, Name
FROM HumanResources.Department
WHERE GroupName = 'Manufacturing'
albeit in the WHERE clause not the SELECT clause. It looks like I cannot use = the equality operator to compare expressions in my SELECT clause as they are being wrongly interpreted as assignment.
How do I include a Boolean equality column comparison equivalent to FirstEvent.TitleID = SecondEvent.TitleID AS titlesSameCheck in my SELECT clause?

Answer 1

Like this:
case when FirstEvent.TitleID = SecondEvent.TitleID then 1 else 0 end as titlesSameCheck

Answer 2

You cannot use the Boolean type directly except in conditional statements (case, where, having, etc.)
Best way to solve your problem is to do something like
select case when x = y then 'true' else 'false' end
The bit type is probably the closest to boolean.
select CAST(case when x = y then 1 else 0 end as bit)
Of course, use whichever two values best represent what you are after.

Answer 3

As the two existing answers state, boolean values can't be returned as a column value. This is documented in the Comparison Operators section:
Unlike other SQL Server data types, a Boolean data type cannot be
specified as the data type of a table column or variable, and cannot
be returned in a result set.
Given that restriction, using CASE to transform the value to something that can be displayed is your best alternative.