Learn Local and Global Temporary Tables in SQL Server

Temporary tables are a useful feature in SQL for storing intermediate results or temporary data during complex operations. They are particularly handy in scenarios where you need to manipulate or process data in multiple steps or within different sessions.

Local Temporary Tables

• Prefixed with a single pound sign (#).
• Scoped to the session that creates them, meaning they are only visible within the session that creates them.
• Automatically dropped when the session ends or when the last query referencing them completes.
• Useful for individual sessions, such as within stored procedures, dynamic SQL batches, or any scenario where you need temporary storage specific to a single session.

DROP TABLE IF EXISTS #TempLocalTable;
CREATE TABLE #TempLocalTable (
    ID INT,
    Name VARCHAR(50)
)

Global Temporary Tables

• Prefixed with a double-pound sign (##).
• Visible to all sessions, meaning they can be accessed and manipulated by any session.
• Dropped when the last session referencing them ends.
• Useful for sharing data among multiple sessions, such as in multi-step processes where different sessions perform various tasks and need access to shared temporary data.

DROP TABLE IF EXISTS ##GlobalTempTable;
CREATE TABLE ##GlobalTempTable (
    ID INT,
    Name VARCHAR(50)
);

Usage scenarios

• Complex Operations: When you have a complex operation that involves multiple steps and requires storing intermediate results.
• Session-Specific Data: When you need temporary storage specific to a single session, such as within stored procedures or dynamic SQL batches.
• Sharing Data Among Sessions: When you need to share temporary data among multiple sessions, such as in multi-step processes involving different sessions. Global temporary tables provide a way to share data across sessions while ensuring each session has its own instance of the data.

Temporary tables in SQL Server offer several benefits

1. Intermediate Data Storage: Temporary tables provide a way to store intermediate results during complex operations. This can be particularly useful in multi-step processes where data needs to be manipulated or processed iteratively.
2. Session Isolation: Local temporary tables are visible only to the session that creates them. This isolation ensures that each session can work with its own set of temporary data without interference from other sessions.
3. Reduced Resource Consumption: Temporary tables can help reduce resource consumption on the server by storing data temporarily rather than permanently. Once the temporary data is no longer needed, it is automatically dropped, freeing up resources.
4. Enhanced Performance: In some cases, using temporary tables can improve query performance by allowing you to materialize intermediate results and reuse them in subsequent steps rather than recalculating the same data multiple times.
5. Complex Query Simplification: Temporary tables can simplify complex queries by breaking them down into smaller, more manageable steps. This can improve code readability and maintainability, making it easier to debug and optimize queries.
6. Scope Control: Global temporary tables can be useful for sharing data among multiple sessions, providing a way to maintain a shared state across different connections or processes.
7. Security and Permissions: Temporary tables inherit permissions from the user who creates them, allowing for fine-grained control over who can access and manipulate the temporary data.