How to Efficiently Retrieve Missing Employee Records Using SQL?

Retrieving Missing Data Using "SELECT * WHERE NOT EXISTS"

The user intends to extract all records from the "employees" table where specific cells do not exist in the "eotm_dyn" table. To achieve this, the user employs the following query:

SELECT * from employees WHERE NOT EXISTS (SELECT name FROM eotm_dyn)

However, this query consistently yields no results despite knowing that approximately 20 names are missing.

Understanding the Issue

The error lies in the failure to join the two tables in the query. As it stands, the query simply evaluates the existence of names in the "eotm_dyn" table without referencing the "employees" table. This will always return false unless the "eotm_dyn" table is empty.

Solution: Joining the Tables

To join the tables and filter out the missing names, modify the query as follows:

SELECT  *
FROM    employees e
WHERE   NOT EXISTS
        (
        SELECT  null 
        FROM    eotm_dyn d
        WHERE   d.employeeID = e.id
        )

In this modified query, a LEFT JOIN is implicitly performed between the "employees" and "eotm_dyn" tables based on the common "employeeID" field. The WHERE clause then utilizes NOT EXISTS to filter out any employee records whose names (or employeeID) do not exist in the "eotm_dyn" table.

Alternate Approach: LEFT JOIN and Filtering NULL Values

Alternatively, one could use a LEFT JOIN and filter out NULL values as follows:

SELECT  *
FROM    employees e
LEFT JOIN
        eotm_dyn d ON e.employeeID = d.employeeID
WHERE   d.name IS NULL

This approach may be less efficient than using NOT EXISTS, but it offers a straightforward method of retrieving the missing data.

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