Sql Union And Union All Complete Guide

SQL UNION and UNION ALL: A Detailed Explanation with Important Information

Overview:

Purpose:

• Combinational Retrieval: Both UNION and UNION ALL are used to combine the result sets of two or more SELECT statements into a single dataset.
• Consolidation of Data: They enable the consolidation of data from different tables or even different databases, creating a unified view for analysis and reporting.

Syntax:

1. UNION:

   SELECT column1, column2, ...
   FROM table1
   UNION
   SELECT column1, column2, ...
   FROM table2;
   

2. UNION ALL:

   SELECT column1, column2, ...
   FROM table1
   UNION ALL
   SELECT column1, column2, ...
   FROM table2;
   

Both UNION and UNION ALL require that the SELECT statements have the same number of columns and that corresponding columns have compatible data types.

Differences and Characteristics:

1. Elimination of Duplicates:

   • UNION: Removes duplicate rows from the final result set. This means that if there are identical rows across the different SELECT statements, they appear only once in the output.
   • UNION ALL: Includes all rows from both SELECT statements, including duplicates. Therefore, rows that are identical in both tables will appear twice in the result set.

2. Performance:

   • UNION: Generally slower than UNION ALL because it requires an additional operation to check for and remove duplicate rows.
   • UNION ALL: Faster as it directly merges the results of the SELECT statements without duplicate elimination.

3. Order By Clause:

   • The ORDER BY clause applies to the combined results of the SELECT statements, not to the individual query results.
   • Only one ORDER BY clause can be used in a UNION or UNION ALL statement and it must be placed at the end.

4. Use in Complex Queries:

   • Both UNION and UNION ALL can be used in conjunction with subqueries and other SQL clauses like WHERE, GROUP BY, and HAVING.

Important Considerations:

1. Column Names:

   • The names of columns in the result set for UNION or UNION ALL are taken from the first SELECT statement.
   • If you want specific column names in the result, use the AS keyword after the column name in the first SELECT statement.

2. Data Types:

   • Ensure that the data types of corresponding columns across the SELECT statements are compatible. SQL typically performs implicit type conversion but may result in errors if data types are not compatible.
   • Use explicit type casting (e.g., CAST(column_name AS data_type)) when necessary.

3. Sorting and Filtering:

   • You can sort the combined result using ORDER BY.
   • Filtering should be done within each SELECT statement to reduce unnecessary computational overhead.

4. Limitations:

   • UNION and UNION ALL cannot be used with GROUP BY and HAVING clauses directly in the combined statement unless used within subqueries.
   • They can only be used with SELECT statements that have the same number of columns.

Example:

Suppose we have two tables:

1. employees_us | employee_id | first_name | last_name | salary | |-------------|------------|-----------|--------| | 1 | John | Doe | 50000 | | 2 | Jane | Smith | 60000 |

2. employees_ca | employee_id | first_name | last_name | salary | |-------------|------------|-----------|--------| | 3 | Emily | Johnson | 55000 | | 2 | Jane | Smith | 60000 |

Combining using UNION:

SELECT employee_id, first_name, last_name, salary
FROM employees_us
UNION
SELECT employee_id, first_name, last_name, salary
FROM employees_ca;

Result: | employee_id | first_name | last_name | salary | |-------------|------------|-----------|--------| | 1 | John | Doe | 50000 | | 2 | Jane | Smith | 60000 | | 3 | Emily | Johnson | 55000 |

Combining using UNION ALL:

SELECT employee_id, first_name, last_name, salary
FROM employees_us
UNION ALL
SELECT employee_id, first_name, last_name, salary
FROM employees_ca;

Result: | employee_id | first_name | last_name | salary | |-------------|------------|-----------|--------| | 1 | John | Doe | 50000 | | 2 | Jane | Smith | 60000 | | 3 | Emily | Johnson | 55000 | | 2 | Jane | Smith | 60000 |

Notice how the row (2, Jane, Smith, 60000) appears twice in the UNION ALL result due to its presence in both tables.

Practical Applications:

• Cross-Table Reporting: Combining data from different sources for comprehensive reports.
• Merging Similar Data: When working with multiple datasets that share similar structures and need to be merged together.
• Data Deduplication: Utilizing UNION to eliminate duplicate entries across datasets.
• Handling Large Datasets: Using UNION ALL when dealing with large volumes of data where duplicate elimination is not critical for performance reasons.