What does coalesce mean in sql

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.

As a result, you can get different results depending on the isolation level of the query. As an alternative, you can rewrite the query to push the subquery into a subselect as shown in the following example:. Data type determination of the resulting expression is different. 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:.

This example uses the AdventureWorks database. As defined by Wikipedia. Introduced by the creator of the relational database model, E. SQL null is a state, not a value. This usage is quite different from most programming languages, where null value of a reference means it is not pointing to any object.

During the expression evaluation process the NULL values are replaced with the user-defined value. The SQL Coalesce function evaluates the arguments in order and always returns first non-null value from the defined argument list. And we can see that we encounter a NULL value while processing string concatenation operation.

The result is not a combination of the firstname, null, and last name. It allows handling the behavior of the NULL value. We now see that the full name gets displayed with a space in the middle, for NULL values.

In this way, it is possible to efficiently customize the column values. Some products may not have the summary but the other do. In this case, you can use the COALESCE function to return the product summary, and if the product summary is not provided, you get the first 50 characters from the product description. You can use the CONCAT function to add the … to the end of the excerpt to make it more meaningful to users that the text they are reading is just the excerpt and there is more content if they click the read more link.

Suppose you need to calculate the net price of all products and you came up with the following query :.