Unions in MySQL 

In MySQL, “UNION’  is used to combine the results of two or more `SELECT` queries into a single result set. The queries must have the same number of columns and compatible data types. Unions in MySQL are the elegant fusion mechanism that stitches together result sets from multiple SELECT queries into a single, seamless stream, enabling analysts to consolidate data across tables, databases, or even servers as if they were one unified source, all while preserving performance and precision.

Introduced to handle scenarios where data lives in parallel structures—like monthly sales partitioned into sales_jan, sales_feb—or when combining similar datasets from different systems, UNION removes the need for complex joins or temporary tables by vertically stacking rows.

The basic form, UNION, automatically eliminates duplicate rows thereby delivering a clean, deduplicated output ideal for reporting, while UNION ALL retains every row, prioritizing speed over uniqueness in high-volume analytics. For compatibility, all SELECT statements must share identical column counts, data types, and order, with column names inherited from the first query—aliases in subsequent queries are ignored but can clarify intent.

ORDER BY and LIMIT apply to the final result, not individual queries, so wrapping in a subquery becomes essential for per-segment sorting. In practice, unions power dynamic dashboards that aggregate daily active users across regional databases, merge legacy and new CRM exports for migration testing, or compile cross-department KPIs into executive summaries. Performance scales efficiently with indexed keys and filtered pre-unions, avoiding full table scans.

In lecture terms, think of UNION as database-level concatenation with intelligence: it’s not just stacking but harmonizing disparate truths into one coherent narrative. Advanced use includes recursive unions for hierarchical data traversal, like org charts or bill-of-materials explosion.

Mastery of unions transforms fragmented data environments into fluid, queryable wholes, making MySQL a true enterprise analytics engine.

The main points to consider when using UNION are:

– UNION’  automatically removes duplicate rows.

– **UNION ALL” includes all duplicate rows in the result.

Syntax:-

SELECT column_list FROM table1

UNION [ALL]

SELECT column_list FROM table2;

Example:-

sql

SELECT id, name FROM customers

UNION

SELECT id, name FROM suppliers;

This query combines the results from the `customers` and `suppliers` tables, removing duplicates. If you want to keep duplicates, use `UNION ALL` instead.

Practical use of Unions in MySQL:

• Combine monthly sales tables (sales_jan, sales_feb) into yearly report with UNION ALL
• Merge customer lists from multiple regional databases for global CRM view
• Aggregate login events from web and mobile apps into single activity stream
• Compile product catalogs from legacy and new systems during platform migration
• Generate full-site search index by unioning content from posts, products, and pages
• Create audit trails merging insert/update/delete logs across microservices
• Build cross-platform analytics joining Android, iOS, and web user behavior
• Consolidate A/B test results from variant tables into unified conversion report
• Merge backup snapshots with live data for disaster recovery validation
• Union survey responses from multiple forms into single sentiment analysis dataset
• Combine IoT sensor streams from different device fleets into real-time dashboard
• Generate compliance reports unioning access logs from on-prem and cloud servers
• Create master patient records by unioning EHR data from multiple hospital systems
• Build financial close packages merging GL, AP, and AR subledger extracts
• Union competitor pricing scrapes from daily crawls into trend analysis table