UNION vs UNION ALL in SQL

Understanding the nuanced distinctions between the UNION and UNION ALL procedures in SQL is essential for effective data s

UNION and UNION ALL are SQL operators utilised to combine two or more result sets. This enables the execution of multiple SELECT statements, retrieval of the desired results, and subsequent combination into a final, unified dataset.

Combine our two tables via the UNION method. 

SELECT employee_id, name, department FROM employees_2023 

UNION

SELECT employee_id, name, department FROM employees_2024; 

Duplicate records are excluded from this result.

Combine our two tables by employing UNION ALL.

SELECT employee_id, name, department FROM employees_2023 UNION ALL 

SELECT employee_id, name, department FROM employees_2024 

keeps all records, including duplicates

 

In the realm of SQL, the capacity to combine data from various queries is a key necessity, and SQL provides robust methods such as UNION and UNION ALL to facilitate this process.

UNION and UNION ALL share identical fundamental requirements for the data being merged. 

• Each SELECT statement must retrieve an equal number of columns for successful combination. 
• The columns obtained should maintain a consistent order across all SELECT statements. 
• The columns retrieved must be of similar data types.

UNION and UNION ALL serve to concatenate data sets and determine the retention of duplicates. Where UNION executes a deduplication process prior to presenting the final results, UNION ALL on the other hand, preserves all duplicates and provides the complete, concatenated results.