SQL Temporary Tables

A temporary table in MS SQL Server is a special type of table that is used to store data temporarily. Temporary tables are created in the tempdb database and are available only for the duration of the session or transaction in which they were created.

There are two types of temporary tables in MS SQL Server:

• Local temporary tables: These tables are prefixed with a single pound (#) symbol and are visible only to the session that created them. Local temporary tables are automatically dropped when the session that created them ends.
• Global temporary tables: These tables are prefixed with a double pound (##) symbol and are visible to all sessions. Global temporary tables are dropped automatically when the last session using them ends.

Temporary tables are useful in situations where you need to store intermediate results that are used in multiple steps of a query or stored procedure. For example, if you need to perform a series of calculations on a large data set, you could store the intermediate results in a temporary table and use them in subsequent steps of the calculation, rather than recalculating them each time.

To create a temporary table in MS SQL Server, you can use the CREATE TABLE statement with the # or ## prefix.

An example

CREATE TABLE #my_temp_table (
                                                            id INT PRIMARY KEY,
                                                            name VARCHAR(50)
                                                           );

Once you have created a temporary table, you can insert data into it using the INSERT INTO statement, and query the data using SELECT, UPDATE, and DELETE statements as you would with a regular table.

Here are some factors to consider when deciding whether to use temporary tables or table expressions:

• Performance: Temporary tables require disk I/O and can incur overhead for creating and dropping the table. Table expressions are in-memory structures that are generated on the fly and can often provide better performance than temporary tables.
• Data volume: If your result set contains a large amount of data, using a temporary table may be more efficient than using a table expression. This is because temporary tables can be indexed, whereas table expressions cannot.
• Complexity: If your query or stored procedure involves multiple steps or complex logic, using temporary tables may be a better option than using table expressions. Temporary tables can simplify the query by breaking it down into smaller, more manageable parts.
• Scope: If you need to share the result set with other sessions or processes, using a temporary table may be a better option than using a table expression. Temporary tables can be accessed by other sessions or processes, whereas table expressions are only visible within the current query.