How to Fix Error 1222 (21000) "The Used SELECT Statements Have a Different Number of Columns" When Using UNION in MySQL

This article explains the causes of Error 1222 in MySQL and provides solutions to fix it when using the UNION operator.

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The UNION Conundrum in MySQL

When working with MySQL’s UNION operator, you’re essentially trying to combine multiple result sets into one. It’s like merging several trains onto a single track - all cars must align perfectly, or you’ll get a derailment. Error 1222 occurs when your SELECT statements in a UNION operation don’t have matching column counts, causing MySQL to reject the entire operation.

This error typically appears when you’re combining data from different tables or queries that weren’t designed with unification in mind. The database needs each SELECT statement in your UNION to return exactly the same number of columns, with compatible data types in corresponding positions.

Understanding the Core Problem

Why Column Counts Must Match

MySQL requires uniform structure across all SELECT statements in a UNION because:

  • It needs to determine the structure of the final result set
  • Client applications expect consistent column counts when processing results
  • The database engine must know how to map data from different sources

Common Scenarios That Trigger the Error

-- Example 1: Different column counts
SELECT id, name, email FROM users
UNION
SELECT id, username FROM admins;  -- Missing email column

-- Example 2: Mismatched in complex queries
SELECT product_id, name, price FROM current_products
UNION
SELECT item_code, description FROM archived_products;  -- Missing price column

Practical Solutions for Column Matching

Aligning Column Counts Exactly

The most straightforward fix is ensuring each SELECT returns the same number of columns:

-- Fixed version of Example 1
SELECT id, name, email FROM users
UNION
SELECT id, username, '' FROM admins;  -- Added empty string for email

Using NULL Placeholders for Missing Data

When you don’t have equivalent data in all tables, use NULL as a placeholder:

SELECT id, name, email, phone FROM customers
UNION
SELECT id, username, NULL, NULL FROM system_users;  -- Missing contact info

Creating Consistent Aliases

For better readability and maintenance, use consistent column aliases:

SELECT
    user_id AS id,
    full_name AS name,
    email_address AS email
FROM legacy_users
UNION
SELECT
    id,
    CONCAT(first_name, ' ', last_name) AS name,
    email
FROM new_users;

Advanced UNION Techniques

Combining Different Data Structures

When dealing with fundamentally different tables, create a unified structure:

SELECT
    product_id AS id,
    product_name AS name,
    price,
    'current' AS source
FROM current_inventory
UNION
SELECT
    item_id AS id,
    item_description AS name,
    unit_price AS price,
    'archive' AS source
FROM archived_items;

Using Derived Tables for Complex Cases

Wrap queries in derived tables to normalize column counts:

SELECT a, b, c FROM (
    SELECT col1 AS a, col2 AS b, col3 AS c FROM table1
    UNION
    SELECT colA AS a, colB AS b, NULL AS c FROM table2
) AS combined_data;

Handling UNION with Aggregations

When using aggregate functions, ensure consistent grouping:

SELECT
    department,
    COUNT(*) AS employee_count,
    NULL AS avg_salary  -- Placeholder
FROM employees
GROUP BY department
UNION
SELECT
    department,
    NULL,  -- Placeholder
    AVG(salary) AS avg_salary
FROM salaries
GROUP BY department;

Debugging and Prevention Strategies

Systematic Error Checking

  1. Count columns in each SELECT separately before combining
  2. Test each query independently to verify column counts
  3. Use DESCRIBE or EXPLAIN to examine result structures

Development Best Practices

  • Document expected column structures for complex unions
  • Implement incremental testing - build unions gradually
  • Use consistent naming conventions across related tables
  • Consider creating views for frequently used unions

Visual Alignment Technique

Format your UNION queries vertically to spot mismatches:

SELECT id, name, email, phone        FROM users
UNION
SELECT id, username, NULL, extension FROM staff  -- Clearly aligned
UNION
SELECT id, nickname, email, mobile   FROM contacts;

Special Considerations

UNION vs UNION ALL

Remember that UNION ALL has the same column count requirements but preserves duplicates:

-- Both versions require matching column counts
SELECT a, b FROM table1
UNION ALL
SELECT x, y FROM table2;

Data Type Compatibility

While we’re focusing on column counts, remember that corresponding columns must also have compatible data types:

SELECT int_column, text_column FROM tableA
UNION
SELECT text_column, int_column FROM tableB;  -- Will cause runtime errors

Conclusion

Error 1222’s complaint about mismatched column counts in UNION operations is MySQL’s way of enforcing data structure integrity. While it might seem restrictive, this requirement ensures predictable results and prevents subtle data corruption issues.

The solutions range from simple placeholder values to more sophisticated query restructuring. The key is recognizing that UNION isn’t just about combining data—it’s about creating a coherent, unified structure from disparate sources. Whether you use NULL placeholders, consistent aliasing, or derived tables, maintaining column count symmetry is essential.

Remember that well-structured unions lead to more maintainable code. Taking the time to properly align your SELECT statements will pay dividends in query reliability and future debugging efforts. When in doubt, break down complex unions into their component parts and verify each one independently before combining them.