Group by with union MySQL select query

sql

best-practices

group-by

union

byAlex Kataev·Dec 6, 2024

To efficiently group combined results from various selects, envelop a UNION inside a subquery and then use GROUP BY in the outer query:

SELECT 
  col, 
  COUNT(*),  //counts the records
  SUM(val)   //sums up the values
FROM (       //this is our subquery
    SELECT col, val FROM tbl1  //record set 1
    UNION ALL    //reunites separated twins
    SELECT col, val FROM tbl2  //record set 2
) sub  
GROUP BY col;  //groups it like a team huddle!

This code ensures seamless amalgamation of data merged from different tables (tbl1, tbl2) using similar columns (col and val). Using UNION ALL preserves all records, omit ALL if you prefer to filter duplicate records.

Best practices and gotchas

Time to delve into the nuts and bolts of UNION and GROUP BY queries. Here are some essential strategies to employ for clarity, accuracy, and performance:

Subqueries: Subqueries are akin to inception. A query within a query can bring structure and readability to your SQL code.
Table Aliases: Make your queries more understandable with table aliases, effectively circumventing ambiguity present in JOIN clauses.
JOIN Clauses: When merging tables, always use an INNER JOIN or explicit joins over implied syntax. Finally, a way to bring tables to their junction point.
Aggregation Rules: For each non-aggregated column in SELECT, make sure it's mentioned in GROUP BY clause. It keeps SQL standards happy and you, bug-free!

Mission critical measures

Sidestep slip-ups

Spelling Check: A misspelled table or column name can crash your query. Remember, SQL doesn't come with spellcheck!
SELECT Clause Consistency: Always consign non-aggregated columns in your SELECT statement in the GROUP BY clause. Because SQL doesn't like loose ends!

Ensure successful execution

Sample Data Validation: Test the waters first with sample data to confirm expected results. Better safe than sorry!

Optimize for success

UNION vs UNION ALL: Choose wisely, young padawan. UNION filters duplicates, while UNION ALL does not.
A comprehensive count: Use COUNT(*) to include all rows, nulls included. Admit it, sometimes NULLs deserve some love too!

Error prevention

Regular Validation: Keep checking your query against database rules and ensure there are no syntax errors lurking beneath the surface.
GROUP BY: A GROUP BY clause is like a magnifying glass to your data. Make sure all fields facilitate your data aggregation goal.