When working with SQL Server, you often need a temporary place to store data for calculations, intermediate results, or complex queries. This is where temporary tables come in handy. These are special tables that exist only for a limited time and are automatically cleaned up by the system. There are two main types: local and global.

Local Temporary Tables (#table_name)

Imagine you’re working on a complex problem and need a personal scratchpad to jot down notes and calculations that only you will see. That’s essentially what a local temporary table is.

• Creation: You create them by prefixing the table name with a single hash symbol, like #YourTempTable.
• Scope: They are exclusive to your current database session. This means if you open another query window or another user connects, they won’t be able to see or access your local temporary table.
• Lifespan: Your local temporary table is automatically deleted the moment your session (e.g., your SSMS query window) closes or disconnects from the database. It’s like your scratchpad disappearing once your work session is over.

Global Temporary Tables (##table_name)

Now, picture a shared whiteboard in an office. Everyone can see it, write on it, and erase from it. A global temporary table functions similarly.

• Creation: These tables are identified by a double hash symbol prefix, such as ##SharedTempTable.
• Scope: Unlike local temp tables, global temporary tables are accessible to all active sessions on the SQL Server Management Studio instance. Any connected user can see and interact with it.
• Lifespan: A global temporary table has a slightly more complex lifespan. It persists as long as at least one active session is referencing it. It’s only automatically deleted when the very last connection that has been interacting with that specific table closes. Think of it as the whiteboard remaining in place until the last person who used it leaves the office and turns out the lights.

Both local and global temporary tables are powerful tools for managing transient data, helping to simplify queries and improve performance by breaking down complex operations into manageable steps. Choosing between them depends on whether the temporary data needs to be isolated to a single user or shared across multiple users and sessions.