In SQL Server, both table variables and temporary tables are used to store and manipulate data within a query. However, they have some key differences in terms of their usage, scope, and performance characteristics. Let's explore these differences and when to use each.

Table Variables

Scope: Table variables are only visible within the scope of the batch, stored procedure, or function in which they are defined. They have a limited scope and are typically destroyed when the batch or procedure execution is completed.

Declaration: You declare a table variable using the DECLARE statement. For example:

DECLARE @MyTableVar TABLE (ID INT, Name NVARCHAR(50));

Transaction Isolation: Table variables are always in-memory structures and are not logged. This means they are not affected by transaction rollbacks or logs, making them a good choice for temporary, small datasets within a single transaction.

Performance: Table variables are optimized for smaller datasets. They do not have statistics, indexes, or parallelism, which can impact their performance for larger datasets. They are suitable for holding a few rows of data.

Usages: Use table variables when dealing with small, temporary datasets that won't be used across multiple stored procedures or functions, and when transaction isolation is not a concern.

Temporary Tables

Scope: Temporary tables are visible across multiple batches, stored procedures, and even different sessions. They can be global or local, and their scope is broader than table variables.

Declaration: Temporary tables are declared using the CREATE TABLE statement with a # prefix for local temporary tables (visible only within the current session) and ## for global temporary tables (visible across different sessions). For example:

CREATE TABLE #MyTempTable (ID INT, Name NVARCHAR(50));

Transaction Isolation: Temporary tables are logged and can participate in transactions. This can be an advantage when you need data persistence and reliability across transactions.

Performance: emporary tables can be indexed and have statistics, making them suitable for larger datasets. They are better for complex queries and join operations.

Usages: Use temporary tables when dealing with larger datasets, when data needs to persist beyond the scope of a single batch or transaction, and when you need transaction isolation and reliability.

Conclusion

Choosing between a table variable and a temporary table depends on the specific use case. Table variables are preferable for small to medium-sized datasets and simple operations, especially when memory usage and logging overhead are concerns. Temporary tables, on the other hand, are more suitable for larger datasets and complex operations, where the benefits of indexing, statistics, and transaction support outweigh the additional overhead.