PostgreSQL is a powerful, open-source relational database management system that offers a wide range of features for creating and managing tables. One of the most useful features is the ability to create a new table based on the results of a SELECT statement, also known as "CREATE TABLE AS SELECT" (CTAS) or "CREATE TABLE WITH SELECT" (CTWS). This feature allows you to quickly and easily create a new table with the same structure and data as an existing table, without having to manually define the table's columns and data types.
In this article, we will take a closer look at how to use the CTAS and CTWS features in PostgreSQL, and provide some code examples to help you get started.
Syntax
The basic syntax for creating a table using the CTAS or CTWS feature is as follows:
CREATE TABLE new_table_name
AS SELECT * FROM existing_table_name;
This will create a new table called "new_table_name" with the same structure and data as the existing table called "existing_table_name". The * in the SELECT statement is used to select all columns from the existing table.
Alternatively, you can specify a list of columns to select instead of using the * wildcard, like this:
CREATE TABLE new_table_name
AS SELECT column1, column2, column3 FROM existing_table_name;
You can also include a WHERE clause to filter the rows that are inserted into the new table. For example:
CREATE TABLE new_table_name
AS SELECT * FROM existing_table_name WHERE column1 = 'value1';
You can also specify additional options such as the name of the schema, indexes and constraints and the like. For example:
CREATE TABLE new_schema.new_table_name
AS SELECT * FROM existing_schema.existing_table_name
WITH (OIDS = FALSE);
This creates a new table new_table_name in the new_schema schema with the same data as existing_table_name in existing_schema and does not include OIDs(Object identifiers)
Examples
Here are a few examples of how to use the CTAS and CTWS feature in PostgreSQL:
Example 1: Create a new table with the same structure and data as an existing table
CREATE TABLE backup_employees
AS SELECT * FROM employees;
This example creates a new table called "backup_employees" with the same structure and data as the existing "employees" table.
Example 2: Create a new table with a subset of columns from an existing table
CREATE TABLE employee_names
AS SELECT first_name, last_name FROM employees;
This example creates a new table called "employee_names" with only the "first_name" and "last_name" columns from the "employees" table.
Example 3: Create a new table with a filtered subset of data from an existing table
CREATE TABLE active_employees
AS SELECT * FROM employees WHERE status = 'active';
This example creates a new table called "active_employees" with all columns from the "employees" table, but only the rows where the "status" column is equal to 'active'.
As you can see, the CT
Additional Features
In addition to the basic CTAS and CTWS functionality, PostgreSQL also provides a few additional features that can be useful when creating new tables.
IF NOT EXISTS
When creating a new table, it's possible that the table already exists in the database. To avoid errors in this case, you can use the "IF NOT EXISTS" clause, which will only create the table if it does not already exist.
CREATE TABLE IF NOT EXISTS backup_employees
AS SELECT * FROM employees;
WITH NO DATA
Another option when creating a new table is to use the "WITH NO DATA" clause, which will create a new table with the same structure as the selected table, but without any data. This can be useful if you want to create a new table with the same structure as an existing table, but without copying any data.
CREATE TABLE backup_employees
WITH (OIDS = FALSE) AS SELECT * FROM employees WITH NO DATA;
CONCURRENTLY
When creating a new table with a large amount of data, it can take a long time to complete the operation. To avoid this, you can use the "CONCURRENTLY" clause, which allows you to create the new table in the background while other queries can continue to use the existing table.
CREATE TABLE backup_employees
AS SELECT * FROM employees CONCURRENTLY;
TEMPORARY TABLES
PostgreSQL also supports the creation of temporary tables, which are only visible to the current session and are automatically dropped when the session is closed. To create a temporary table, you can use the "TEMP" or "TEMPORARY" keyword, like this:
CREATE TEMPORARY TABLE temp_employees
AS SELECT * FROM employees;
Conclusion
The CTAS and CTWS features in PostgreSQL provide a powerful and convenient way to create new tables based on existing data. With the ability to select specific columns, filter rows, and specify additional options, you can easily create new tables that are customized to your needs. Additionally, the additional features such as "IF NOT EXISTS", "WITH NO DATA", "CONCURRENTLY" and "TEMPORARY" tables provide more flexibility and fine-grained control over the table creation process. With the help of these features, you can quickly and easily create new tables that are tailored to your specific needs.
Popular questions
- What is the basic syntax for creating a table using the CTAS or CTWS feature in PostgreSQL?
- The basic syntax for creating a table using the CTAS or CTWS feature is:
CREATE TABLE new_table_name
AS SELECT * FROM existing_table_name;
- Can you specify a list of columns to select when using the CTAS or CTWS feature in PostgreSQL?
- Yes, instead of using the
*wildcard to select all columns, you can specify a list of columns to select. For example:
CREATE TABLE new_table_name
AS SELECT column1, column2, column3 FROM existing_table_name;
- Can you include a WHERE clause when using the CTAS or CTWS feature in PostgreSQL?
- Yes, you can include a WHERE clause to filter the rows that are inserted into the new table. For example:
CREATE TABLE new_table_name
AS SELECT * FROM existing_table_name WHERE column1 = 'value1';
- Can you create a new table with the same structure but without data using CTAS in PostgreSQL?
- Yes, you can use the "WITH NO DATA" clause to create a new table with the same structure as the selected table, but without any data. For example:
CREATE TABLE backup_employees
WITH (OIDS = FALSE) AS SELECT * FROM employees WITH NO DATA;
- Can you create a new table in the background while other queries can continue to use the existing table using CTAS in PostgreSQL?
- Yes, you can use the "CONCURRENTLY" clause to create a new table in the background while other queries can continue to use the existing table. For example:
CREATE TABLE backup_employees
AS SELECT * FROM employees CONCURRENTLY;
Tag
PostgreSQL
