JSON (JavaScript Object Notation) is a popular data interchange format that is widely used in web applications. With the release of SQL Server 2016, Microsoft introduced native support for JSON, allowing developers to store and query JSON data directly in the database.

One of the most important tasks when working with JSON data is to ensure that the documents being entered are well-structured and valid. To address this, SQL Server provides a useful function called ISJSON(). This function can be used to validate whether a given string represents a valid JSON document or not.

The ISJSON() function is deterministic, meaning that it will always return the same result for the same input. This makes it easy to use in procedural contexts, such as in a stored procedure or a check constraint.

Here is an example of how to use the ISJSON() function:

DECLARE @json NVARCHAR(4000)
SET @json = N'{
"id": 1,
"name": "A green door",
"price": 12.50,
"color": "Green",
"type": "bricks",
"tags": ["home", "green"]
}'

IF (ISJSON(@json) = 1)
BEGIN
    PRINT 'The JSON document is valid'
END

In the above code, we first declare a variable @json and assign a JSON document to it. We then use the ISJSON() function to check if the document is valid. If the function returns 1, it means that the document is valid and we print a message to indicate that.

In addition to using the ISJSON() function in procedural contexts, we can also use it to enforce data integrity in SQL Server tables. For example, we can add a check constraint to a table to ensure that a column contains valid JSON data:

CREATE TABLE MyJSONTable(
    Id INT,
    jsonColumn VARCHAR(MAX) NULL CHECK (ISJSON(jsonColumn) > 0)
);

In the above code, we create a table called MyJSONTable with a column called jsonColumn. We add a check constraint to the column using the ISJSON() function to ensure that only valid JSON data can be inserted into the column.

It is worth noting that the ISJSON() function returns a NULL if the input is NULL. This can be useful to handle cases where the JSON document is optional.

Working with JSON documents in SQL Server 2016 opens up new possibilities for storing and querying structured data. The ISJSON() function provides a simple yet powerful tool to validate JSON documents and enforce data integrity. As you explore more complex scenarios with JSON data, feel free to share your experiences and questions in the comments section below.