4/9/2023 0 Comments Create a join in idatabase![]() How to Use SQL Server Coalesce to Work with NULL Values Be aware when creating tables whether you want to allow NULL values or not.īy allowing NULL values you introduce other issues that you may need to face.When dealing with NULL values don't forget about the ISNULL function.Internally a value of NULL is an unknown value and therefore SQL Serverĭoes not equate an unknown value being equal to another unknown value.ĭesign decision is to not allow NULL values when creating tables and therefore you will not run into SummaryĪs we have seen from the above examples joining NULL values does not work.Įven though you have two NULL values SQL Server does not treat these as the same But be aware of this potential performance issue. In some cases there are no other options, so you need to do what you need toĭo. Or on the left side of your WHERE clause, because SQL Server needs to interrogateĮach value and therefore may negate the use of the index. In general it is not a good idea to use functions in your joins A lot of this willĪlso depend on the indexes you have setup and how the indexes are used for the This function in the join clause will slow down the query. Most of the records there is some value, but there are a few records where the TrimĪND ( a.Trim = b.Trim OR ( a.Trim IS NULL AND b.Trim IS NULL ) ) Potential Performance IssuesĪlthough the ISNULL function is handy, if you are doing large joins having Here is that we want to join the tables based on Make, Model and Trim. The first thing we will do is show the data in these two tables. INSERT INTO dbo.CarModels VALUES ('Nissan','Altima',NULL) INSERT INTO dbo.CarModels VALUES ('Nissan','Altima','4-door HYBRID Sedan') INSERT INTO dbo.CarModels VALUES ('Nissan','Altima','4-door 3.5 SL Sedan') INSERT INTO dbo.CarModels VALUES ('Nissan','Altima','4-door 3.5 SE Sedan') INSERT INTO dbo.CarModels VALUES ('Nissan','Altima','4-door 2.5 S Sedan') INSERT INTO dbo.CarModels VALUES ('Nissan','Altima','2-door 3.5 SE Coupe') INSERT INTO dbo.CarModels VALUES ('Nissan','Altima','2-door 2.5 S Coupe') INSERT INTO dbo.CarModels VALUES ('Nissan','Altima','') INSERT INTO dbo.Orders VALUES ('Nissan','Altima',NULL) INSERT INTO dbo.Orders VALUES ('Nissan','Altima','') ![]() INSERT INTO dbo.Orders VALUES ('Nissan','Altima','4-door 3.5 SE Sedan') INSERT INTO dbo.Orders VALUES ('Nissan','Altima','2-door 2.5 S Coupe')
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