In this tutorial, you have learned how to use the ALTER TABLE ALTER COLUMN statement to change the type of a column. The following statement adds the USING clause to the above statement: ALTER TABLE assetsĪLTER COLUMN asset_no TYPE INT USING asset_no:: integer Code language: SQL (Structured Query Language) ( sql ) HINT: You might need to specify "USING asset_no::integer". PostgreSQL issued an error and a very helpful hint: ERROR: column "asset_no" cannot be cast automatically to type integer To change the data type of the asset_no column to integer, you use the following statement: ALTER TABLE assetsĪLTER COLUMN asset_no TYPE INT Code language: SQL (Structured Query Language) ( sql ) The following statement changes the data types of description and location columns from TEXT to VARCHAR: ALTER TABLE assetsĪLTER COLUMN description TYPE VARCHAR Code language: SQL (Structured Query Language) ( sql ) To change the data type of the name column to VARCHAR, you use the following statement: ALTER TABLE assetsĪLTER COLUMN name TYPE VARCHAR Code language: SQL (Structured Query Language) ( sql ) ( 'UPS', '10002', 'Server room', '') Code language: SQL (Structured Query Language) ( sql ) INSERT INTO assets( name,asset_no,location,acquired_date) Let’s create a new table named assets and insert some rows into the table for the demonstration. The expression after the USING keyword can be as simple as column_name::new_data_type such as price::numeric or as complex as a custom function. In case the cast fails, PostgreSQL will issue an error and recommends you provide the USING clause with an expression for the data conversion. If you omit the USING clause, PostgreSQL will cast the values to the new ones implicitly. The USING clause specifies an expression that allows you to convert the old values to the new ones. PostgreSQL allows you to convert the values of a column to the new ones while changing its data type by adding a USING clause as follows: ALTER TABLE table_nameĪLTER COLUMN column_name TYPE new_data_type USING expression Ĭode language: SQL (Structured Query Language) ( sql ) In this syntax, you add a comma ( ,) after each ALTER COLUMN clause. To change the data types of multiple columns in a single statement, you use multiple ALTER COLUMN clauses like this: ALTER TABLE table_nameĪLTER COLUMN column_name1 TYPE new_data_type,ĪLTER COLUMN column_name2 TYPE new_data_type, The SET DATA TYPE and TYPE are equivalent. Third, supply the new data type for the column after the TYPE keyword.Second, specify the name of the column that you want to change the data type after the ALTER COLUMN clause.First, specify the name of the table to which the column you want to change after the ALTER TABLE keywords.Let’s examine the statement in a greater detail: To change the data type of a column, you use the ALTER TABLE statement as follows: ALTER TABLE table_name ALTER COLUMN column_name TYPE new_data_type Code language: CSS ( css ) It will drop any additional attributes of the view, though.Summary: this tutorial shows you step by step how to change the data type of a column by using the ALTER TABLE statement. This will never delete any data of the underlying tables. You have to DROP the old and CREATE a new view. Materialized views are an integral feature of pretty much all advanced database systems. However, if you want to change data types of resulting columns (like in the case at hand), CREATE OR REPLACE VIEW is not possible. ALTER MATERIALIZED VIEW Materialized view REFRESH MATERIALIZED VIEW REFRESH MATERIALIZED VIEW CONCURRENTLY A materialized view is a database object which stores the result of a precalculated database query and makes it easy to refresh this result as needed. Use CREATE OR REPLACE VIEW to change the query - it will preserve any additional attributes. The ALTER VIEW statement can only change auxiliary attributes of a view. If you change underlying objects, you may need to change depending views, too. (That's why you can alter views with an ALTER TABLE command.) You can GRANT privileges to it, add comments or even define column defaults (useful for a rule ON INSERT TO my_view DO INSTEAD.). Views are implemented as special tables with a rule ON SELECT TO my_view DO INSTEAD. Detailed explanationĪ view in PostgreSQL is not just an "alias to subquery". You can change or drop that constraint any time without touching depending objects like views and without forcing Postgres to write new rows in the table due to the change of type (which isn't always necessary any more in modern version of Postgres). If you really want to enforce a maximum length, create a CHECK constraint: ALTER TABLE monkeyĪDD CONSTRAINT monkey_name_len CHECK (length(name) < 101) Read about these data types in the manual. To avoid the problem altogether use the data type text or varchar / character varying without a length specifier instead of character varying(n).
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