How to Efficiently Find Missing Records Using SQL\'s `NOT EXISTS`?

SELECT * WHERE NOT EXISTS: Finding Missing Records

The SQL query "SELECT * from employees WHERE NOT EXISTS (SELECT name FROM eotm_dyn)" aims to retrieve all rows from the "employees" table where their names are not present in the "eotm_dyn" table. However, this query is incomplete and will return no results.

To complete the query, it is necessary to join the two tables on a common field, typically a unique identifier field. Assuming the tables are joined on the "employeeID" field, the corrected query becomes:

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

The "NOT EXISTS" subquery checks for the absence of any rows in the "eotm_dyn" table where the "employeeID" matches the current row's "id" in the "employees" table. If no such rows exist, the outer query will return the entire row from the "employees" table. This approach ensures that only employees without entries in the "eotm_dyn" table are selected.

Alternatively, the query could be written using a LEFT JOIN with a subsequent filter for NULL values:

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

However, this method may be less efficient than using the NOT EXISTS subquery, especially for large databases.

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