CHOOSE() in SQL Server

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While I know I don't utilize most of the features available in SQL Server, I like to think I'm at least aware that those features exist.

This week I found a blind-spot in my assumption however. Even though it shipped in SQL Server 2012, the SQL Server CHOOSE function is a feature that I think I'm seeing for the first time this past week.

CHOOSE is CASE

CHOOSE returns the n-th item from a comma-delimited list.

Whenever learning a new feature in SQL Server I try to think of a good demo I could build to test out the functionality. In this case the immediate example that came to mind was building something that would provide a lookup of values:

SELECT 
    [key],
    [value],
    [type],
    CHOOSE(type+1,'null','string','int','boolean','array','object') AS JsonType
FROM
    OPENJSON(N'{
        "Property1":null,
        "Property2":"a",
        "Property3":3,
        "Property4":false,
        "Property5":[1,2,"3"],
        "Property6":{
            "SubProperty1":"a"
        }
    }');

In this case, the OPENJSON function returns a "type" field that indicates the datatype of that particular JSON property's value. The issue is that the "type" column is numeric and I can never remember what type of data each number represents.

The above query solves this by using CHOOSE to create a lookup of values. Since OPENJSON returns results starting with 0, we need to use type+1 in order to get the 1-based CHOOSE function to work correctly:

2019-06-03-21-08-58

If we look at the CHOOSE function's scalar operator properties in the execution plan, we'll see that this function is just a fancy alias for a more verbose CASE statement:

[Expr1000] = Scalar Operator(
    CASE WHEN (CONVERT_IMPLICIT(int,OPENJSON_DEFAULT.[type],0)+(1))=(1) 
    THEN 'null' 
    ELSE 
        CASE WHEN (CONVERT_IMPLICIT(int,OPENJSON_DEFAULT.[type],0)+(1))=(2) 
        THEN 'string' 
        ELSE 
            CASE WHEN (CONVERT_IMPLICIT(int,OPENJSON_DEFAULT.[type],0)+(1))=(3) 
            THEN 'int' 
            ELSE 
                CASE WHEN (CONVERT_IMPLICIT(int,OPENJSON_DEFAULT.[type],0)+(1))=(4) 
                THEN 'boolean' 
                ELSE 
                    CASE WHEN (CONVERT_IMPLICIT(int,OPENJSON_DEFAULT.[type],0)+(1))=(5) 
                    THEN 'array' 
                    ELSE 
                        CASE WHEN (CONVERT_IMPLICIT(int,OPENJSON_DEFAULT.[type],0)+(1))=(6) 
                        THEN 'object' 
                        ELSE NULL END 
                    END 
                END 
            END 
        END 
    END
)

The Set-Based Way

I think one of the reasons I've never used CHOOSE is because I would hate typing up all of those lookup values and trapping them in a SELECT statement, never to be used again.

Previously, I would have stored the lookup values in table and joined them with the OPENJSON results to accomplish the same end result:

DROP TABLE IF EXISTS #JsonType;
CREATE TABLE #JsonType
(
    Id tinyint,
    JsonType varchar(20),
    CONSTRAINT PK_JsonTypeId PRIMARY KEY CLUSTERED (Id)
);

INSERT INTO #JsonType VALUES (0,'null');
INSERT INTO #JsonType VALUES (1,'string');
INSERT INTO #JsonType VALUES (2,'int');
INSERT INTO #JsonType VALUES (3,'boolean');
INSERT INTO #JsonType VALUES (4,'array');
INSERT INTO #JsonType VALUES (5,'object');

SELECT 
    j.[key],
    j.[value],
    j.[type],
    t.JsonType
FROM
    OPENJSON(N'{
        "Property1":null,
        "Property2":"a",
        "Property3":3,
        "Property4":false,
        "Property5":[1,2,"3"],
        "Property6":{
                        "SubProperty1":"a"
                    }
    }') j
    INNER JOIN #JsonType t
        ON j.[type] = t.Id

While more initial setup is involved with this solution, it's more flexible long-term. With a centralized set of values, there's no need to update the CHOOSE function in all of your queries when you can update the values in a single lookup table.

And while I didn't bother performance testing it, by virtue of being a scalar function, CHOOSE will probably perform worse in many real-world scenarios when compared to the table-based lookup approach (eg. large datasets, parallel plans, etc...).

CHOOSE What Works For You

I'm not surprised that it took me this long to learn about the CHOOSE function: while a simplified way to write certain CASE statements, I can't think of many (any?) scenarios where I would prefer to use it over a CASE or a lookup-table solution.