COALESCE (Transact-SQL)

Άρθρο
05/23/2023

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 SQL database 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.

Transact-SQL syntax conventions

Syntax

COALESCE ( expression [ ,...n ] )

Arguments

expression
Is an expression of any type.

Return Types

Returns the data type of expression with the highest data type precedence. If all expressions are nonnullable, the result is typed as nonnullable.

Remarks

If all arguments are NULL, COALESCE returns NULL. At least one of the null values must be a typed NULL.

Comparing COALESCE and CASE

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:

CASE  
WHEN (expression1 IS NOT NULL) THEN expression1  
WHEN (expression2 IS NOT NULL) THEN expression2  
...  
ELSE expressionN  
END

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.

For example, when the code COALESCE((subquery), 1) is executed, the subquery is evaluated twice. As a result, you can get different results depending on the isolation level of the query. For example, the code can return NULL under the READ COMMITTED isolation level in a multi-user environment. To ensure stable results are returned, use the SNAPSHOT ISOLATION isolation level, or replace COALESCE with the ISNULL function. As an alternative, you can rewrite the query to push the subquery into a subselect as shown in the following example:

SELECT CASE WHEN x IS NOT NULL THEN x ELSE 1 END  
FROM  
(  
SELECT (SELECT Nullable FROM Demo WHERE SomeCol = 1) AS x  
) AS T;

Comparing COALESCE and ISNULL

The ISNULL function and the COALESCE expression have a similar purpose but can behave differently.

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.
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.
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:
```
USE tempdb;  
GO  
-- This statement fails because the PRIMARY KEY cannot accept NULL values  
-- and the nullability of the COALESCE expression for col2   
-- evaluates to NULL.  
CREATE TABLE #Demo   
(   
  col1 INTEGER NULL,   
  col2 AS COALESCE(col1, 0) PRIMARY KEY,   
  col3 AS ISNULL(col1, 0)   
);   

-- This statement succeeds because the nullability of the   
-- ISNULL function evaluates AS NOT NULL.  

CREATE TABLE #Demo   
(   
  col1 INTEGER NULL,   
  col2 AS COALESCE(col1, 0),   
  col3 AS ISNULL(col1, 0) PRIMARY KEY   
);  
```
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.
ISNULL takes only two parameters. By contrast COALESCE takes a variable number of parameters.

Examples

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.

SELECT Name, Class, Color, ProductNumber,  
COALESCE(Class, Color, ProductNumber) AS FirstNotNull  
FROM Production.Product;

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.

SET NOCOUNT ON;  
GO  
USE tempdb;  
IF OBJECT_ID('dbo.wages') IS NOT NULL  
    DROP TABLE wages;  
GO  
CREATE TABLE dbo.wages  
(  
    emp_id        TINYINT   IDENTITY,  
    hourly_wage   DECIMAL   NULL,  
    salary        DECIMAL   NULL,  
    commission    DECIMAL   NULL,  
    num_sales     TINYINT   NULL  
);  
GO  
INSERT dbo.wages (hourly_wage, salary, commission, num_sales)  
VALUES  
    (10.00, NULL, NULL, NULL),  
    (20.00, NULL, NULL, NULL),  
    (30.00, NULL, NULL, NULL),  
    (40.00, NULL, NULL, NULL),  
    (NULL, 10000.00, NULL, NULL),  
    (NULL, 20000.00, NULL, NULL),  
    (NULL, 30000.00, NULL, NULL),  
    (NULL, 40000.00, NULL, NULL),  
    (NULL, NULL, 15000, 3),  
    (NULL, NULL, 25000, 2),  
    (NULL, NULL, 20000, 6),  
    (NULL, NULL, 14000, 4);  
GO  
SET NOCOUNT OFF;  
GO  
SELECT CAST(COALESCE(hourly_wage * 40 * 52,   
   salary,   
   commission * num_sales) AS money) AS 'Total Salary'   
FROM dbo.wages  
ORDER BY 'Total Salary';  
GO

Here's the result set.

Total Salary  
------------  
10000.00  
20000.00  
20800.00  
30000.00  
40000.00  
41600.00  
45000.00  
50000.00  
56000.00  
62400.00  
83200.00  
120000.00  
  
(12 row(s) affected)

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:

Name         Color      ProductNumber  
------------ ---------- -------------  
Socks, Mens  NULL       PN1278  
Socks, Mens  Blue       PN1965  
NULL         White      PN9876

We then run the following COALESCE query:

SELECT Name, Color, ProductNumber, COALESCE(Color, ProductNumber) AS FirstNotNull   
FROM Products ;

Here's the result set.

Name         Color      ProductNumber  FirstNotNull  
------------ ---------- -------------  ------------  
Socks, Mens  NULL       PN1278         PN1278  
Socks, Mens  Blue       PN1965         Blue  
NULL         White      PN9876         White

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.

D: Complex Example

The following example uses COALESCE to compare the values in three columns and return only the non-null value found in the columns.

CREATE TABLE dbo.wages  
(  
    emp_id        TINYINT   NULL,  
    hourly_wage   DECIMAL   NULL,  
    salary        DECIMAL   NULL,  
    commission    DECIMAL   NULL,  
    num_sales     TINYINT   NULL  
);  
INSERT INTO dbo.wages (emp_id, hourly_wage, salary, commission, num_sales)  
VALUES (1, 10.00, NULL, NULL, NULL);  
  
INSERT INTO dbo.wages (emp_id, hourly_wage, salary, commission, num_sales)  
VALUES (2, 20.00, NULL, NULL, NULL);  
  
INSERT INTO dbo.wages (emp_id, hourly_wage, salary, commission, num_sales)  
VALUES (3, 30.00, NULL, NULL, NULL);  
  
INSERT INTO dbo.wages (emp_id, hourly_wage, salary, commission, num_sales)  
VALUES (4, 40.00, NULL, NULL, NULL);  
  
INSERT INTO dbo.wages (emp_id, hourly_wage, salary, commission, num_sales)  
VALUES (5, NULL, 10000.00, NULL, NULL);  
  
INSERT INTO dbo.wages (emp_id, hourly_wage, salary, commission, num_sales)  
VALUES (6, NULL, 20000.00, NULL, NULL);  
  
INSERT INTO dbo.wages (emp_id, hourly_wage, salary, commission, num_sales)  
VALUES (7, NULL, 30000.00, NULL, NULL);  
  
INSERT INTO dbo.wages (emp_id, hourly_wage, salary, commission, num_sales)  
VALUES (8, NULL, 40000.00, NULL, NULL);  
  
INSERT INTO dbo.wages (emp_id, hourly_wage, salary, commission, num_sales)  
VALUES (9, NULL, NULL, 15000, 3);  
  
INSERT INTO dbo.wages (emp_id, hourly_wage, salary, commission, num_sales)  
VALUES (10,NULL, NULL, 25000, 2);  
  
INSERT INTO dbo.wages (emp_id, hourly_wage, salary, commission, num_sales)  
VALUES (11, NULL, NULL, 20000, 6);  
  
INSERT INTO dbo.wages (emp_id, hourly_wage, salary, commission, num_sales)  
VALUES (12, NULL, NULL, 14000, 4);  
  
SELECT CAST(COALESCE(hourly_wage * 40 * 52,   
   salary,   
   commission * num_sales) AS DECIMAL(10,2)) AS TotalSalary   
FROM dbo.wages  
ORDER BY TotalSalary;

Here's the result set.

Total Salary  
------------  
10000.00  
20000.00  
20800.00  
30000.00  
40000.00  
41600.00  
45000.00  
50000.00  
56000.00  
62400.00  
83200.00  
120000.00

Κοινή χρήση μέσω

COALESCE (Transact-SQL)

Syntax

Arguments

Return Types

Remarks

Comparing COALESCE and CASE

Comparing COALESCE and ISNULL

Examples

A. Running a simple example

B. Running a complex example

C: Simple Example

D: Complex Example

See Also

Σχόλια

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