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SQL Lesson 20: Combining Datasets with UNION

Introduction & Core Concept

While JOIN operations connect tables horizontally by matching relational keys, database reporting frequently requires combining results from multiple independent queries vertically into a single unified list.

To stack datasets vertically, SQL provides the UNION and UNION ALL set operators.

SELECT name FROM hr_employees
UNION ALL
SELECT name FROM finance_employees;
Strict Set Operator Rules

1. Column Count Match: Every SELECT query in the union statement must project the exact same number of columns.

2. Datatype Compatibility: Columns must share compatible datatypes in the exact same index order.

3. Header Naming: The final returned dataset derives its column headers entirely from the aliases defined in the *first* query.

UNION vs. UNION ALL (Performance Critical)
  • UNION (Implicit De-duplication): Merges rows and performs a CPU-intensive sort across the entire combined dataset to remove duplicate rows.
  • UNION ALL (High-Speed Concatenation): Appends rows directly without sorting or de-duplication checks. It is incredibly fast and efficient!
    • Why & Where We Use It
  • Why We Use It: Consolidates disparate regional databases, merges historical and current tables, and combines distinct customer segment queries.
  • Where We Use It: Master employee company rosters, global search bars across multiple entities, and consolidated financial ledgers.
    • Real-World Example

    StaffCorp needs a consolidated master roster combining employee names and salaries from both their Scranton and Kathmandu branch locations into a single unified report.

    Best Practices: What to Do & What NOT to Do
  • What to Do: Standardize on using UNION ALL in production code unless you have an explicit business requirement to eliminate duplicates.
  • What NOT to Do: Never attempt to place ORDER BY clauses inside individual sub-queries of a union statement. ORDER BY can only appear once at the very end of the entire unioned query!
    • Syntax & Pro Tips
    SELECT name, city FROM employees_east UNION ALL SELECT name, city FROM employees_west;
    Interactive Sandboxed Terminal (Preloaded DB Schema: STAFFCORP)
    SQL Query WorkspaceSQLite v3.45 (WASM Mode)
    QUICK INSERT:
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