How to Create and Reuse SQL Temporary Tables

TL;DR

Create a temporary table by using the standard CREATE TABLE syntax with a pound sign before the table name, then populate it with inserted values or query results. Temporary tables can be queried repeatedly, making them useful for storing a smaller dataset, avoiding repeated joins and calculations, and supporting stored procedures that need reusable intermediate results.

Transcript

what's going on everybody welcome back to another sql tutorial today we are looking at temp tables and if you can guess it based off of the name they're kind of like temporary tables and we create them very much the same way we're going to do create table it's just a little bit different and you can hit off of this temp table multiple times which y... Read More

Key Insights

• A SQL temporary table is created with standard CREATE TABLE syntax, with a pound sign placed at the beginning of its name. Its columns and data types are declared in the same general way as those of a regular table.
• Temporary-table data can be inserted with an INSERT INTO statement followed by explicit values. Once inserted, those records can be retrieved with an ordinary SELECT statement, allowing the temporary structure to behave much like a regular table during subsequent work.
• A temporary table can be referenced multiple times after it has been created. This differs from the demonstrated use of a common table expression, which must be included whenever it is needed, and from repeated subqueries that may need to be written again.
• Temporary tables can be populated directly from an existing table by inserting the results of a SELECT query. This method is more practical than entering records individually when the goal is to capture a substantial subset of existing data for later analysis.
• A smaller temporary dataset can reduce repeated work against an extremely large source table. After the relevant records are stored, later queries can use the prepared subset instead of repeatedly applying complex joins, window functions, and other operations to the full source.
• Aggregated query results can be stored in a temporary table for additional analysis. The example prepares job title, employee count, average age, and average salary values from joined employee tables, then makes those calculated results available without rebuilding the join and aggregations.
• Temporary tables are commonly useful inside stored procedures because they can preserve intermediate results during execution. Repeated procedure runs may fail if a temporary table with the requested name already exists, so the creation process should account for that possibility.
• DROP TABLE IF EXISTS makes temporary-table creation repeatable by removing an existing table before creating a new one. Placing this statement before CREATE TABLE helps stored procedures and reusable scripts run smoothly when they are executed again in the same working context.

Questions & Answers

Q: How do you create a temporary table in SQL?

Create a temporary table with the usual CREATE TABLE statement, but place a pound sign at the beginning of the table name. Then define each required column and its data type inside parentheses, just as you would for a regular table. The example defines fields for an employee identifier, job title, and salary before executing the creation statement.

Q: How do you insert data into a SQL temporary table?

Use INSERT INTO with the temporary table name, followed by VALUES when adding records directly. The tutorial inserts an employee identifier, a job title, and a salary, then uses SELECT to confirm that the record was stored. Temporary tables can also receive the results of a SELECT query, which is more practical for transferring existing datasets.

Q: How can a temporary table be populated from another table?

Write an INSERT INTO statement targeting the temporary table, then follow it with a SELECT query that retrieves data from the source table. The tutorial copies employee salary data this way rather than entering every record separately. This pattern is useful when a large existing dataset needs to be filtered or prepared for repeated downstream queries.

Q: Why use a temporary table instead of a CTE or subquery?

A temporary table can be created and then referenced repeatedly without recreating its defining query each time. In the tutorial, a common table expression must be included whenever it is needed, while a subquery may need to be written multiple times within a query. A temporary table keeps its prepared data available for several later operations.

Q: How can temporary tables improve SQL query runtime?

Temporary tables can store the relevant subset or calculated output of a costly query before additional analysis begins. Later queries can read those prepared values instead of repeatedly processing a very large source table, rebuilding joins, or recalculating aggregates. The tutorial states that this can reduce runtime and processing demands when the same intermediate dataset is used repeatedly.

Q: How can temporary tables store aggregated query results?

Create a temporary table with columns that match the expected aggregated output, then insert the results of a query containing joins and aggregate calculations. The tutorial stores job title, employees per job, average age, and average salary. Further calculations can then use those stored results without repeatedly running the original joins and aggregation logic.

Q: Why are temporary tables useful in stored procedures?

Stored procedures often use temporary tables to hold intermediate query results that later statements need. This allows the procedure to separate expensive data preparation from subsequent calculations. However, running the procedure again can produce an error if a temporary table with the same name is still present, so repeatable procedures should handle any existing table before creation.

Q: How do you prevent errors when recreating a temporary table?

Place a DROP TABLE IF EXISTS statement before the CREATE TABLE statement and specify the temporary table name. If the table already exists, SQL deletes it before attempting to create the replacement. If it does not exist, the script continues. This pattern allows a stored procedure or reusable query to run repeatedly without failing because of an existing temporary table.

Summary & Key Takeaways

• Temporary tables are created much like regular SQL tables, except their names begin with a pound sign. After defining columns and data types, users can insert individual values or select records from another table. The resulting temporary table can then be queried repeatedly without rebuilding its contents for every operation.

• A practical use for temporary tables is storing a relevant subset of data from a very large source table. Later queries can work against that prepared subset instead of repeatedly processing the original table. This approach is especially useful when subsequent work involves joins, window functions, aggregations, or other complex calculations.

• Temporary tables can hold aggregated results produced from joined tables, such as job titles, employee counts, average ages, and average salaries. Stored procedures often use them for intermediate data. Adding DROP TABLE IF EXISTS before creation prevents errors when the procedure runs again while a temporary table with the same name still exists.