![]() ![]() 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. Code language: SQL (Structured Query Language) ( sql ) 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 ) Alternatively, you can add constraints later (see below) after you've filled in the new column correctly.Summary: this tutorial shows you step by step how to change the data type of a column by using the ALTER TABLE statement. Keep in mind however that the default value must satisfy the given constraints, or the ADD will fail. In fact all the options that can be applied to a column description in CREATE TABLE can be used here. You can also define constraints on the column at the same time, using the usual syntax:ĪLTER TABLE products ADD COLUMN description text CHECK (description '') To avoid a potentially lengthy update operation, particularly if you intend to fill the column with mostly nondefault values anyway, it may be preferable to add the column with no default, insert the correct values using UPDATE, and then add any desired default as described below. However, if the default value is volatile (e.g., clock_timestamp()) each row will need to be updated with the value calculated at the time ALTER TABLE is executed. Instead, the default value will be returned the next time the row is accessed, and applied when the table is rewritten, making the ALTER TABLE very fast even on large tables. From PostgreSQL 11, adding a column with a constant default value no longer means that each row of the table needs to be updated when the ALTER TABLE statement is executed. ![]()
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