Using a Case Statement in a query seems to work fine as long as the only case you want to match to is a string. But if you want to put ">" greater than (date or number), etc, SSMS automatically adds apostrophes around your statement (below) as if it were a string.
How can I get it to accept "greater than date" or "greater than number", etc??
There are two types of CASE expression:
The simple CASE expression compares an expression to a set of simple expressions to determine the result.
The searched CASE expression evaluates a set of Boolean expressions to determine the result.
You want a searched CASE expression. This means that the WHEN should come immediately after the CASE keyword.
CASE WHEN SoldDate <= SubjectDate THEN ...
There are two types of CASE expressions (from msdn):
A Simple CASE compares the specified expression for EQUIVALENCY ONLY
SELECT FieldName CASE WHEN 'Blah' THEN 'Foo' ELSE 'Bah' END
A Searched CASE can do inequalities too but has more verbose syntax:
SELECT CASE WHEN FieldName >= 'Blah' THEN 'Foo' ELSE 'Bah' END
You are trying to use the Simple syntax and need to use the Searched syntax by explicitly writing out the fieldname and comparison for each evaluation.
Your CASE statement syntax is incorrect. WHEN should come directly after CASE
This:
CASE SoldDate WHEN ...
Should be this:
CASE WHEN SoldDate <= SubjectDate THEN SoldFor ELSE EstSalePrice END
Additional Information
Take a look at the MSDN docs for additional examples of CASE syntax.
Related
how can I convert to T-Sql this one?
IIf([ESSValue]<>0,Int([ESSValue]*100),"")
I think the following pretty much does what you want:
select coalesce(cast(EssValue * 100 as int), 0)
Here is the thinking. The comparison to zero is unimportant, because 0 times any value is going to be zero. The iif() returns an integer (I think) because the "then" argument is an integer; the empty string gets converted to zero.
I'm not 100% certain about the last statements with regard to MS Access, but that is how iif() works in SQL Server.
I should add. Although I don't approve of iif() for conditional expressions (because case is the standard and more powerful), SQL Server does support it. So you could write:
IIf([ESSValue]<>0, cast([ESSValue]*100 as int), '')
Note: As I mentioned earlier, the '' will be converted to 0.
CASE WHEN ESSValue <> 0
THEN CAST(ESSValue * 100 AS INT)
ELSE NULL
END as fieldname
For case expression the default is NULL if doesn't meet any condition, so you dont really need the ELSE condition
When I write an sql case when statement(s), does it function like an if, if ,..if logic or if, else-if , else-if, else logic?
i.e if the condition matches case #1, will it still evaluate the other cases?
Any self-respecting database will short circuit, but you didn't mention which database you are using. So, try it: select case when 1=1 then 1 else 1/0 end and see.
The case statement evaluates the when conditions in order. It stops at the first condition that evaluates to true. This is ANSI standard behavior and, as far as I know, all databases support the case statement in this way.
Is it possible to use a comparison operator in a CONVERT or CAST function?
I've got a statement that looks like this:
SELECT
...
CASE field
WHEN 'Y' THEN 1 # If Y then True
ELSE 0 # Anything else is False
END
...
FROM ...
A similar thing happens for a few fields, so I would like to change it to a shorter version:
SELECT
...
CONVERT(BIT, field = 'Y')
...
FROM ...
But MSSQL is giving an error Incorrect syntax near '='.
My interpretation of the help is that it should work:
CONVERT ( data_type [ ( length ) ] , expression [ , style ] )
expression: expression { binary_operator } expression
binary_operator: Is an operator that defines the way two expressions are combined to yield a single result. binary_operator can be an arithmetic operator, the assignment operator (=), a bitwise operator, a comparison operator, a logical operator, the string concatenation operator (+), or a unary operator.
comparison operator: ( = | > | < | >= | <= | <> | != | !< | !> )
I ran a few tests and got these results:
SELECT CONVERT(BIT, 0) // 0
SELECT CONVERT(BIT, 1) // 1
SELECT CONVERT(BIT, 1+2) // 1
SELECT CONVERT(BIT, 1=2) // Error
SELECT CONVERT(BIT, (1=2)) // Error
SELECT CONVERT(BIT, (1)=(2)) // Error
I think you are misinterpreting the documentation for CONVERT. There is nothing in the documentation for CONVERT that states it will handle an expression that makes use of the comparison operators, only that it accepts an expression. It turns out that CONVERT does not handle every valid SQL expression. At the very least it cannot handle the results of an expression that uses a comparison operator.
If you check the documentation for Operators, you'll see that the comparison operators (which is what you want = to be, in this case) return a Boolean data type, and are used in WHERE clauses and control-of-flow statements. From the documentation for Operators:
The result of a comparison operator has the Boolean data type, which has three values:
TRUE, FALSE, and UNKNOWN. Expressions that return a Boolean data type are known as
Boolean expressions.
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.
...
Expressions with Boolean data types are used in the WHERE clause to
filter the rows that qualify for the search conditions and in
control-of-flow language statements such as IF and WHILE...
That helps to explain why SQL like SELECT 1=2 is invalid SQL, because it would create a result set with a Boolean data type, which the documentation says is not allowed. That also explains why the CASE WHEN construct is necessary, because it can evaluate the comparison operators and return a single value of a data type that SQL Server can return in a result set.
Furthermore, if you look at the documentation for CONVERT, you'll see that Boolean is not supported in either CAST or CONVERT (see the table towards the middle of the page, there is no Boolean data type in there).
For your purposes, I think you're stuck using CASE WHEN. If it helps you can write it all on one line:
CASE WHEN field = 'Y' THEN 1 ELSE 0 END
Alternatively, you could create a UDF to handle the CASE expression (something like dbo.func_DoCase(field, 'Y', 1, 0)), but personally I would just stick with CASE WHEN.
Given:
The following Select statement:
select case NULL
when NULL then 0
else 1
end
Problem:
I'm expecting this to return 0 but instead it returns 1. What gives?
Generally speaking, NULL is not something you should attempt to compare for equality, which is what a case statement does. You can use "Is NULL" to test for it. There is no expectation that NULL != NULL or that NULL = NULL. It's an indeterminate, undefined value, not a hard constant.
-- To encompass questions in the comments --
If you need to retrieve a value when you may encounter a NULL column, try this instead:
Case
When SomeColumn IS NULL
Then 0
Else 1
End
I believe that should work. As far as your original post is concerned:
Select Case NULL
When NULL then 0 // Checks for NULL = NULL
else 1 // NULL = NULL is not true (technically, undefined), else happens
end
The trouble is that your Case select automatically attempts to use equality operations. That simply doesn't work with NULL.
I was going to add this as a comment to Aaron's answer, but it was getting too long, so I'll add it as another (part of the) answer.
The CASE statement actually has two distinct modes, simple and searched.
From BOL:
The CASE expression has two formats:
The simple CASE expression compares an expression to a set of simple expressions to determine the result.
The searched CASE expression evaluates a set of Boolean expressions to determine the result.
When the simple CASE (your example) does what it describes as comparison it does an equality comparison - i.e. =
This is clarified in the later documentation:
The simple CASE expression operates by comparing the first expression
to the expression in each WHEN clause for equivalency. If these
expressions are equivalent, the expression in the THEN clause will be
returned.
Allows only an equality check.
Because anything = NULL is always false in ANSI SQL (and if you didn't know this, you need to read up on NULLs in SQL more generally, particularly also with the behavior in the other searched comparison - WHERE x IN (a, b, c)), you cannot use NULL in a simple case and have it ever be compared to a value, with a NULL either in the initial expression or in the list of expressions to be compared against.
If you want to check for NULL, you will have to use an IF/ELSE construct or the searched CASE with a full expression.
I agree that it's kind of unfortunate there is no version which supports an IS comparison to make it easier to write:
select case colname
when IS NULL then 0
else 1
end
Which would make writing certain long CASE statements easier:
select case colname
when IS NULL then ''
when 1 then 'a'
when 2 then 'b'
when 3 then 'c'
when 4 then 'd'
else 'z'
end
But that's just wishful thinking...
An option is to use ISNULL or COALESCE:
select case COALESCE(colname, 999999) -- 999999 is some value never used
when 999999 then ''
when 1 then 'a'
when 2 then 'b'
when 3 then 'c'
when 4 then 'd'
else 'z'
end
But it isn't always a great option.
In addition to the other answers, you need to change the syntax for CASE slightly to do this:
SELECT CASE
WHEN NULL IS NULL THEN 0
ELSE 1
END;
Using the value in your syntax implicitly uses an equals comparison. NULL is unknown, and so is NULL = NULL, so with your current code you will always get zero 1 (geez I did it too).
To get the behavior you want, you can use SET ANSI_NULLS ON; however note that this can change other code in ways you may not be able to predict, and the setting is deprecated - so it will stop working at all in a future version of SQL Server (see this SQL Server 2008 doc).
You need to use the IS NULL operator. Standard comparison operators do not work with NULL.
Check out these MSDN articles about Null that may be useful:
IS [NOT] NULL (Transact-SQL)
Null Values
UPDATE Table SET Value = 5 WHERE DateTime = '03:42:34';
Sometimes that DateTime will not exist, though. I'm wondering if there's any way that I can have it dynamically attempt then
UPDATE Table SET Value = 5 WHERE DateTime = '03:42:35';
This is probably not possible, especially since 'DateTime` is a string, but wondered if there might be some way of doing it.
I had thought of taking the first 7 characters of the DateTime and having that match, but that's probably not quite precise enough.
The CASE expression sounds like it could do what you want.
A CASE expression serves a role
similar to IF-THEN-ELSE in other
programming languages.
The optional expression that occurs in
between the CASE keyword and the first
WHEN keyword is called the "base"
expression. There are two basic forms
of the CASE expression: those with a
base expression and those without.
In a CASE without a base expression,
each WHEN expression is evaluated and
the result treated as a boolean,
starting with the leftmost and
continuing to the right. The result of
the CASE expression is the evaluation
of the THEN expression that
corresponds to the first WHEN
expression that evaluates to true. Or,
if none of the WHEN expressions
evaluate to true, the result of
evaluating the ELSE expression, if
any. If there is no ELSE expression
and none of the WHEN expressions are
true, then the overall result is NULL.
http://www.sqlite.org/lang_expr.html
Is this what you want?
UPDATE Table
SET Value = 5
WHERE DateTime =
( SELECT MIN(DateTime)
FROM Table
WHERE DateTime >= '03:42:34'
)
;
I got this working in a straightforward way I feel silly for not thinking of originally:
UPDATE Table SET Value = 5 WHERE DateTime BETWEEN datetime('2020-03-21 03:42:35', '-1 second') and datetime('2020-03-21 03:42:35', '+1 second')