Adding a Date Column to a Temporary Table in SQL

Overview

In this article, we will explore the process of adding a new column with default values to a temporary table in Netezza. The challenge arises when trying to modify an existing temporary table without the necessary administrative privileges to create a permanent table.

Problem Statement

We are working with a temporary table named old_temp_table that contains columns id, gender, start_date, and end_date. We want to add a new column, default_date, which will contain a default date value of '2019-01-01' for all rows.

Solution Overview

The solution involves using the CAST function to convert NULL values in the existing temporary table into the desired data type. Since Netezza doesn’t know the actual data type of the default_date column, we need to cast it to a type that will allow us to store dates.

Using CAST to Convert NULL Values

SELECT id, gender, start_date, end_date,
       CAST(NULL as DATE) as default_date
INTO TEMP TABLE new_temp_table
FROM old_temp_table;

In this code snippet, we use the CAST function to convert NULL values in the default_date column into a DATE type. This will allow us to store dates in the column.

Why Does Netezza Make a Guess?

Netezza doesn’t know what the actual data type of the default_date column is, so it makes an educated guess. In this case, Netezza likely assumes that the default_date column should be an integer, which is not ideal for storing dates.

Avoiding Integer Data Type

To avoid using an integer data type and instead store actual dates, we need to cast the values as a DATE type.

SELECT id, gender, start_date, end_date,
       CAST(NULL as DATE) as default_date
INTO TEMP TABLE new_temp_table
FROM old_temp_table;

Casting to CHAR or VARCHAR

If you want to store dates in a format other than the standard date format used by Netezza (YYYY-MM-DD), you can use the CAST function with the desired data type.

SELECT id, gender, start_date, end_date,
       CAST(NULL as CHAR(10)) as default_date
INTO TEMP TABLE new_temp_table
FROM old_temp_table;

In this example, we cast the values to a character string with a length of 10 characters. This will allow us to store dates in any format.

Creating a Permanent Table

If you have the necessary administrative privileges, you can create a permanent table and modify its structure using SQL commands like ALTER TABLE.

CREATE TABLE new_table (
    id INT,
    gender VARCHAR(50),
    start_date DATE,
    end_date DATE,
    default_date CHAR(10)
);

INSERT INTO new_table (id, gender, start_date, end_date, default_date)
SELECT id, gender, start_date, end_date, '2019-01-01'
FROM old_temp_table;

ALTER TABLE new_table
ADD CONSTRAINT chk_default_date CHECK(default_date LIKE '%2019-%m-%d%');

In this example, we create a permanent table with the desired structure and populate it with data. We also add a check constraint to ensure that the default_date column only contains dates in the format ‘YYYY-MM-DD’.

Conclusion

Adding a new column with default values to a temporary table in Netezza requires some creative problem-solving, but the process is manageable using SQL commands like CAST. By casting values as a specific data type, you can avoid common pitfalls and ensure that your date columns store actual dates.

Last modified on 2023-10-27