How to Find the Missing Values in Your Database With SQL

If you’re working with data in SQL, it’s crucial to identify and deal with missing data.

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In SQL, Null values can indicate missing or unknown data, and they can cause issues if they’re not properly handled.

In this article, we’ll look at how to find the number of null (or missing) values in a column and the missing ratio in SQL.

First, let's define some terms.

• A null value is a placeholder for missing or unknown data. It’s represented in SQL as NULL.
• A missing ratio is the percentage of null values in a column compared to the total number of rows.

You can use the COUNT() function along with a WHERE clause to find the number of null values in a column.

For example, if you have a table called "Customers" with a column called "PhoneNumber", you can use the following query to find the number of null values in the PhoneNumber column:

SELECT
  COUNT(PhoneNumber)
FROM
  Customers
WHERE
  PhoneNumber IS NULL;

This will return the number of rows in the Customers table where the PhoneNumber is null.

To find the missing ratio, you can divide the number of null values by the total number of rows in the table and multiply by 100 to get the percentage.

For example, to find the missing ratio for the PhoneNumber column, you can use the following query:

SELECT
  (COUNT(PhoneNumber) / (SELECT COUNT(*) FROM Customers)) * 100 AS MissingRatio
FROM
  Customers
WHERE
  PhoneNumber IS NULL;

This will return the missing ratio for the PhoneNumber column as a percentage. It's important to note that this method only works to find a single column's missing ratio.

You'll need to use a different approach to find the missing ratio for multiple columns.

One option is to use a CASE statement to create a new column that indicates whether each row has any null values, and then use COUNT() to find the number of rows with null values.

In summary, finding the number of null values and the missing ratio in SQL is a simple process. Using the COUNT() function and a WHERE clause, you can quickly identify null values in a column and calculate the missing ratio as a percentage. This information can help identify missing data and ensure that your data is complete and accurate.

Article created with ChatGPT and reviewed by Mickaël Andrieu