What does isnull mean in sql

The ISNULL function is used to check whether the expression is NULL. If so, it returns the specified replacement value, otherwise it returns the expression itself. The main uses include: 1. Replace NULL values ​​with non-NULL values; 2. Avoid data conversion errors; 3. Display meaningful values.

ISNULL function and its uses

The ISNULL function is a SQL function that checks whether a given expression is NULL. If the expression is NULL, returns the specified replacement value; otherwise, returns the expression itself.

Syntax:

<code class="sql">ISNULL(expression, replacement_value)</code>

Parameters:

• expression: The expression to check Mode.
• replacement_value: The replacement value returned if expression is NULL.

Purpose:

The ISNULL function is mainly used to handle NULL values ​​to prevent data integrity problems. The following are common uses of the ISNULL function:

• Replace NULL values ​​with non-NULL values: The ISNULL function can replace NULL values ​​with predefined non-NULL values ​​to ensure data integrity .
• Avoid data conversion errors: In some cases, NULL values ​​cannot be converted to other data types, such as when performing mathematical operations. The ISNULL function can replace NULL values ​​with placeholders such as 0 to avoid conversion errors.
• Display meaningful values: For some reports or applications, displaying NULL values ​​may not be appropriate or convenient. The ISNULL function replaces a NULL value with a more meaningful text or number, such as "unknown" or "-1".

Example:

The following example will replace NULL's Name column with "Unknown":

<code class="sql">SELECT ISNULL(Name, 'Unknown') AS Name
FROM table_name;</code>

The following example will replace NULL's Name column Replace the Amount column with 0:

<code class="sql">SELECT ISNULL(Amount, 0) AS Amount
FROM table_name;</code>

Note: The

• ISNULL function only works on a single expression. If the expression contains subqueries or JOINs, you may need to use other functions (such as COALESCE).
• The replacement value of the ISNULL function can be any data type.
• If the expression is non-NULL, the ISNULL function returns the expression unchanged.

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