What Is the SQL UNION Operator?

What Is the SQL UNION Operator?

SQL UNION merges the outputs of two or more SELECT statements into a single result set and removes duplicate rows automatically.

Why Use SQL UNION?

SQL UNION is ideal when you need one consolidated list from tables or queries that share the same logical structure but store related data separately.

How Does SQL UNION Work Internally?

Each SELECT produces a temporary table. The database stacks these tables vertically, then applies DISTINCT logic across all columns to discard duplicates.

What Are the Syntax Rules for UNION?

You must write individual SELECTs with matching column counts, compatible data types, and the same column order, then join them with the UNION keyword.

When Should I Use UNION ALL Instead?

Use UNION ALL when duplicate rows are meaningful or performance matters, because UNION ALL skips the de-duplication step and returns rows faster.

Can I Combine More Than Two Queries?

You can chain as many SELECT statements as needed: SELECT … UNION SELECT … UNION SELECT … . Every added query must follow the same column structure.

How Do Column Names Resolve?

The column names from the first SELECT statement become the names of the output columns, regardless of aliases used in later SELECTs.

Do ORDER BY and LIMIT Work with UNION?

Place ORDER BY or LIMIT after the final SELECT block to sort or trim the combined set. Each inner query can have its own ORDER BY only if enclosed in parentheses with LIMIT.

What Are Real-World Use Cases?

Common scenarios include merging archived and live data, unifying region-partitioned tables, or producing a combined list of current and historical records.

How Does UNION Compare to JOIN?

UNION stacks rows vertically (adding more rows), while JOIN merges columns horizontally (adding more attributes). Choose UNION when schemas match and you need more rows.

What Are Performance Considerations?

UNION incurs overhead for duplicate removal. Indexes on participating columns and using UNION ALL when possible improve performance.

Best Practices for Reliable UNION Queries

Explicitly cast mismatched data types, align column lists instead of using SELECT *, and test with UNION ALL first to verify row counts.

How to Debug UNION Errors?

Error messages like “columns of query do not match” indicate column count or data-type mismatches. Compare SELECT lists side by side to resolve quickly.

What’s the Role of Parentheses in Complex UNIONs?

Use parentheses to control precedence when mixing UNION with other set operators (INTERSECT, EXCEPT) or when adding individual ORDER BY clauses.

Key Takeaways

SQL UNION unifies structurally similar queries, removes duplicates, and simplifies reporting across partitioned or archived tables. Follow column-matching rules and prefer UNION ALL for speed when duplicates are acceptable.