When working with SQL Server, developers often come across various types of tables that can be used for different purposes. While physical tables are commonly used for storing and retrieving data, there are other types of tables that can be utilized to optimize SQL queries and stored procedures. In this article, we will explore temporary tables, table variables, and derived tables and discuss their benefits and use cases.
Temporary Tables
Temporary tables are a type of table that are created and used within a specific session or connection. There are two types of temporary tables: local and global. Local temporary tables are visible only to a single connection and are automatically dropped when the connection ends. Global temporary tables, on the other hand, are visible across multiple connections and are dropped when all sessions stop referencing them.
Here is an example of creating a local temporary table:
CREATE TABLE #MyTempTable (
PolicyId INT IDENTITY(1,1) PRIMARY KEY NOT NULL,
LastName VARCHAR(50) NOT NULL
)
Table variables, on the other hand, exist in memory for the duration of a single T-SQL batch. They are declared using a syntax similar to local variable declaration. Here is an example of creating a table variable:
DECLARE @MyTableVariable TABLE (
PolicyId INT IDENTITY(1,1) PRIMARY KEY NOT NULL,
LastName VARCHAR(50) NOT NULL
)
It’s important to note that a table variable is no longer in scope after a GO command is issued from Query Analyzer. However, it can be created at the beginning of a stored procedure and referenced throughout its execution.
Derived Tables
Derived tables are similar to table variables in that they exist in memory only. However, they differ in how they are created and used. A derived table is created when a SELECT query is given a name and joined to from another table. They exist only for the duration of a single T-SQL statement and cannot be referenced outside the statement in which they are created.
Here is an example of using a derived table:
SELECT P.PolicyId, LastName, NewestVehicle
FROM Policy P
INNER JOIN (
SELECT PolicyId, MAX(VehicleYear) AS NewestVehicle
FROM PolicyVehicle PV
GROUP BY PolicyId
) MaxVehicles ON P.PolicyId = MaxVehicles.PolicyId
In this example, a derived table named ‘MaxVehicles’ is created to retrieve the newest vehicle for each policy. It is then joined with the Policy table to retrieve the PolicyId and LastName columns.
Choosing the Right Table Type
When deciding which type of table to use, there are a few things to consider:
- Temporary tables are stored in the SQL Server TEMPDB database and require more IO resources and locking.
- Table variables and derived tables are created in memory.
- Temporary tables perform better for large amounts of data that can be processed using parallelism.
- Table variables are best suited for small amounts of data (typically 100 or less rows) where parallelism would not provide a significant performance improvement.
- Stored procedures cannot insert data into table variables or derived tables.
- Derived tables can only be created from a SELECT statement but can be used within an Insert, Update, or Delete statement.
Understanding the differences and use cases of these table types can greatly optimize your queries and stored procedures. In a future article, we will dive deeper into performance tuning using these table types. Until then, I encourage you to explore and experiment with temporary tables, table variables, and derived tables to enhance the performance of your SQL Server applications.