MS SQL Server - Temporary Tables

Mastering Temporary Tables in MS SQL Server

Temporary tables are a powerful tool in Microsoft SQL Server that enables you to store and manipulate temporary data within your database sessions. In this comprehensive guide, we will explore the world of temporary tables, including their types, creation, usage, and best practices. By the end of this guide, you will be well-prepared to leverage temporary tables for various tasks, from complex data transformations to optimizing query performance.

Understanding Temporary Tables

Temporary tables are a special type of table that exists only for the duration of a database session or a specific batch of queries. They are particularly useful for holding intermediate results, managing complex queries, and sharing data across multiple parts of a query or stored procedure.

Types of Temporary Tables

1. Local Temporary Tables

Local temporary tables are only visible to the session that creates them. They are automatically dropped when the session ends, making them ideal for temporary data storage within a specific session.


Example:
-- Create a local temporary table
CREATE TABLE #TempTable (
  ID INT,
  Name VARCHAR(50)
);
-- Insert data
INSERT INTO #TempTable VALUES (1, 'John'), (2, 'Alice');
-- Query the temporary table
SELECT * FROM #TempTable;

2. Global Temporary Tables

Global temporary tables are visible to all sessions within a SQL Server instance. They are dropped when the last session referencing them ends. Global temporary tables are useful for sharing temporary data across multiple sessions.


Example:
-- Create a global temporary table
CREATE TABLE ##GlobalTempTable (
  ID INT,
  Name VARCHAR(50)
);
-- Insert data
INSERT INTO ##GlobalTempTable VALUES (1, 'John'), (2, 'Alice');
-- Query the global temporary table
SELECT * FROM ##GlobalTempTable;

Creating Temporary Tables

1. Local Temporary Table Creation

To create a local temporary table, you can use the “#” prefix followed by the table name. These tables are only visible to the current session.


Example:
CREATE TABLE #LocalTempTable (
  ID INT,
  Name VARCHAR(50)
);

2. Global Temporary Table Creation

Creating global temporary tables is similar to local temporary tables, but you use “##” as the prefix. Global temporary tables are visible to all sessions but are dropped when the last session ends.


Example:
CREATE TABLE ##GlobalTempTable (
  ID INT,
  Name VARCHAR(50)
);

Practical Use Cases

1. Data Transformation and Aggregation

Temporary tables are ideal for complex data transformations and aggregations. You can store intermediate results during data processing, enabling you to break down complex operations into manageable steps.

2. Query Performance Optimization

Temporary tables can significantly improve query performance. You can precompute and store data in temporary tables, reducing the need for repeated and costly calculations during a query.

3. Sharing Data Across Sessions

Global temporary tables facilitate data sharing across multiple sessions. This is useful when you need to store data temporarily and make it accessible to other users or applications.

Best Practices

1. Properly Index Temporary Tables

If you’re working with large datasets in temporary tables, consider adding appropriate indexes to improve query performance when accessing the temporary data.

2. Clean Up Temporary Tables

Ensure that you drop temporary tables when they are no longer needed. Leaving temporary tables behind can lead to resource contention and increased storage usage.

3. Use Temporary Tables Sparingly

While temporary tables are a valuable tool, avoid overusing them. Excessive temporary table usage can lead to complex and hard-to-maintain code.

Conclusion

Temporary tables in MS SQL Server are a versatile feature that can simplify data manipulation, optimize query performance, and facilitate data sharing across sessions. By understanding their types, creating and using them effectively, and following best practices, you can harness the full potential of temporary tables in your SQL Server database operations.

MS SQL Server – Temporary Tables