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Definition and Usage. The COALESCE() function returns the first non-null value in a list. Syntax. ... SQL Server (starting with 2008), Azure SQL Database, Azure SQL ...
May 24, 2022 · NULL. We can use the COALESCE () function to replace the NULL value with a simple text: SELECT first_name, last_name, COALESCE (marital_status, 'Unknown') FROM persons. The COALESCE () function is used to return the value 'Unknown' only when marital_status is NULL. When marital_status is not NULL, COALESCE () returns the value of the column ...
- Overview
- Return Types
- Remarks
- Comparing COALESCE and CASE
- Comparing COALESCE and ISNULL
- Examples
Applies to: SQL Server Azure SQL Database Azure SQL Managed Instance Azure Synapse Analytics Analytics Platform System (PDW) SQL analytics endpoint in Microsoft Fabric Warehouse in Microsoft Fabric
Evaluates the arguments in order and returns the current value of the first expression that initially doesn't evaluate to NULL. For example, SELECT COALESCE(NULL, NULL, 'third_value', 'fourth_value'); returns the third value because the third value is the first value that isn't null.
Returns the data type of expression with the highest data type precedence. If all expressions are nonnullable, the result is typed as nonnullable.
If all arguments are NULL, COALESCE returns NULL. At least one of the null values must be a typed NULL.
The COALESCE expression is a syntactic shortcut for the CASE expression. That is, the code COALESCE(expression1,...n) is rewritten by the query optimizer as the following CASE expression:
As such, the input values (expression1, expression2, expressionN, and so on) are evaluated multiple times. A value expression that contains a subquery is considered non-deterministic and the subquery is evaluated twice. This result is in compliance with the SQL standard. In either case, different results can be returned between the first evaluation and upcoming evaluations.
The ISNULL function and the COALESCE expression have a similar purpose but can behave differently.
1.Because ISNULL is a function, it's evaluated only once. As described above, the input values for the COALESCE expression can be evaluated multiple times.
2.Data type determination of the resulting expression is different. ISNULL uses the data type of the first parameter, COALESCE follows the CASE expression rules and returns the data type of value with the highest precedence.
3.The NULLability of the result expression is different for ISNULL and COALESCE. The ISNULL return value is always considered NOT NULLable (assuming the return value is a non-nullable one). By contrast,COALESCE with non-null parameters is considered to be NULL. So the expressions ISNULL(NULL, 1) and COALESCE(NULL, 1), although equal, have different nullability values. These values make a difference if you're using these expressions in computed columns, creating key constraints or making the return value of a scalar UDF deterministic so that it can be indexed as shown in the following example:
4.Validations for ISNULL and COALESCE are also different. For example, a NULL value for ISNULL is converted to int though for COALESCE, you must provide a data type.
5.ISNULL takes only two parameters. By contrast COALESCE takes a variable number of parameters.
A. Running a simple example
The following example shows how COALESCE selects the data from the first column that has a nonnull value. This example uses the AdventureWorks2022 database.
B. Running a complex example
In the following example, the wages table includes three columns that contain information about the yearly wages of the employees: the hourly wage, salary, and commission. However, an employee receives only one type of pay. To determine the total amount paid to all employees, use COALESCE to receive only the nonnull value found in hourly_wage, salary, and commission. Here is the result set.
C: Simple Example
The following example demonstrates how COALESCE selects the data from the first column that has a non-null value. Assume for this example that the Products table contains this data: We then run the following COALESCE query: Here is the result set. Notice that in the first row, the FirstNotNull value is PN1278, not Socks, Mens. This value is this way because the Name column wasn't specified as a parameter for COALESCE in the example.
It means that all the remaining arguments are not evaluated at all. The COALESCE function returns NULL if all arguments are NULL. The following statement returns 1 because 1 is the first non-NULL argument. SELECT COALESCE (1, 2, 3); -- return 1 Code language: SQL (Structured Query Language) (sql) The following statement returns Not NULL because ...
Oct 12, 2022 · COALESCE() syntax. The COALESCE() function takes in at least one value (value_1). It will return the first value in the list that is non-null. For example, it will first check if value_1 is null. If not, then it returns value_1. Otherwise, it checks if value_2 is null. The process goes on until the list is complete.
Jan 2, 2024 · To understand how to use COALESCE in SQL, you first need to learn how it works: The function evaluates the expressions received as input from left to right. It returns the first non- NULL expression it encounters. If all expressions are NULL, it returns NULL. In other words, to use the COALESCE SQL function, you have to specify your columns ...
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COALESCE. SQL COALESCE is a useful function that returns the first non-null value from a list of expressions. It takes any number of expressions as arguments, and returns the first non-null expression. If all expressions evaluate to null, then it returns null.
