:: represents CAST operator in PostgreSQL. It is asking us to cast the roll_no values into integer type explicitly. PostgreSQL is even providing us with a hint. Specifically, the error will read − ERROR: column "roll_no" cannot be cast automatically to type integer HINT: You might need to specify "USING roll_no::integer". This is because the conversion of text to integer is not that straightforward for PostgreSQL. Now, let us try to convert the roll_no column back to an integer ALTER TABLE marks However, writing custom SQL for this is quite easy: op.execute ('ALTER TABLE views ALTER COLUMN cols TYPE bit varying USING expr') Of course you have to replace expr with an expression that converts the old data type to the new data. ![]() It simply replaces the digit with the character representing that digit. Unfortunately you need to use raw SQL as alembic doesn't output a USING statement ever when changing types. ![]() Because integer to text conversion does not confuse PostgreSQL. Now, for understanding the cases where we need to use an expression, let us first convert the roll_no column to VARCHAR type. 2 Answers Sorted by: 5 what you are looking to do is CAST it to an integer. We need not use any expression for converting the column type. Now, suppose we want to convert the serial_no column from type INTEGER to BIGINTEGER. Now, let us populate it INSERT INTO marks(serial_no,name, roll_no, marks_obtained, Let us create a new table marks − CREATE TABLE marks( ![]() Let us understand both the cases using an example. However, if you convert a TEXT or a VARCHAR entry to INTEGER, you will not be able to do that by default, unless you provide an expression for converting the existing TEXT/VARCHAR values to an integer. For instance, if you convert a column of type INTEGER to type BIGINTEGER, you need not use any expression for the conversion of existing entries to the new types. It is used when you need to use an expression for converting the existing entries in that column from the current type to the new type. The USING part of the syntax is optional. Please note that altering the type of a column is not recommended generally, especially if your table has a lot of entries already. In order to change the type of a column, the syntax is ALTER TABLE table_nameĪLTER COLUMN column_name TYPE new_data_type USING expression
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