Creating Temporary Tables from Select Statements

Temporary tables are a useful feature in databases that allow you to store data temporarily for a session. They can be used to simplify complex queries, improve performance, and reduce the amount of code needed to accomplish a task. In this tutorial, we will explore how to create temporary tables from select statements without having to specify each column type.

Introduction to Temporary Tables

A temporary table is a table that is visible only to the current session and is automatically dropped when the session is closed. This means that two different sessions can use the same temporary table name without conflicting with each other or with an existing non-temporary table of the same name.

Creating Temporary Tables from Select Statements

To create a temporary table from a select statement, you can use the CREATE TEMPORARY TABLE syntax followed by the AS keyword and the select statement. The basic syntax is as follows:

CREATE TEMPORARY TABLE table_name AS (SELECT * FROM other_table);

This will create a temporary table named table_name with the same columns as the result set of the select statement.

Adding Indexes to Temporary Tables

If you need to add an index to your temporary table, you can do so by specifying the index in the column definition. For example:

CREATE TEMPORARY TABLE temp_table (INDEX(col_2)) ENGINE=MyISAM AS (
  SELECT col_1, col_2, col_3
  FROM mytable
);

This will create a temporary table with an index on the col_2 column.

Choosing the Engine for Temporary Tables

When creating a temporary table, you can choose the engine to use by specifying the ENGINE keyword. For example:

CREATE TEMPORARY TABLE temp1 ENGINE=MEMORY AS (SELECT * FROM table1);

This will create a temporary table using the memory engine.

Note that not all engines support all data types. For example, the memory engine does not support BLOB or TEXT columns.

Example Use Cases

Temporary tables can be used in a variety of scenarios, such as:

• Simplifying complex queries by breaking them down into smaller, more manageable pieces
• Improving performance by reducing the amount of data that needs to be processed
• Reducing the amount of code needed to accomplish a task by reusing temporary tables

For example, suppose you have a query that joins three tables and performs several calculations. You could create a temporary table to store the intermediate results and then use that table in the final query.

CREATE TEMPORARY TABLE temp_table AS (
  SELECT col1, col2, col3
  FROM table1
  JOIN table2 ON table1.id = table2.id
);

SELECT * FROM temp_table
WHERE col1 > 10;

This can make your code easier to read and maintain, and can also improve performance by reducing the amount of data that needs to be processed.

Conclusion

In conclusion, creating temporary tables from select statements is a powerful feature in databases that can simplify complex queries, improve performance, and reduce the amount of code needed to accomplish a task. By using the CREATE TEMPORARY TABLE syntax and specifying the engine and indexes as needed, you can create temporary tables that meet your needs and make your code more efficient and effective.