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Error 44: ILLEGAL_COLUMN

Tip

This error occurs when you reference a column in an illegal context, such as using non-aggregated columns outside of GROUP BY clauses, referencing columns that aren't available in the current query scope, or attempting to use columns in ways that violate ClickHouse's query execution semantics.

Most common causes​

  1. Non-aggregated columns used without GROUP BY

    • Selecting regular columns alongside aggregate functions without including them in GROUP BY
    • Mixing aggregated and non-aggregated columns incorrectly
    • Using columns in HAVING clause that aren't in GROUP BY or aggregate functions

  2. Column scope issues in subqueries

    • Referencing outer query columns in correlated subqueries where not allowed
    • Using columns from inner queries in outer query contexts
    • Incorrect column visibility across query nesting levels

  3. Invalid column references in JOINs

    • Referencing columns from tables not included in the current JOIN scope
    • Using columns before the table is introduced in the FROM/JOIN chain
    • Ambiguous column references when tables have overlapping column names

  4. Array JOIN context violations

    • Using non-array columns as if they were arrays
    • Referencing array-joined columns outside their valid scope
    • Mixing array-joined and regular columns incorrectly

  5. Window function scope issues

    • Using window function results in WHERE or HAVING clauses (not allowed)
    • Referencing window function aliases in contexts where they haven't been evaluated
    • Mixing window functions with aggregates incorrectly

Common solutions​

1. Add missing columns to GROUP BY clause

SELECT 
    user_id,
    count() as total_events
FROM events
GROUP BY date;

-- Solution: Include user_id in GROUP BY
SELECT 
    user_id,
    count() as total_events
FROM events
GROUP BY date, user_id;

2. Use ANY or arbitrary aggregate functions for non-key columns

-- Error: Can't select name without aggregation
SELECT 
    user_id,
    name,
    count() as events
FROM users
GROUP BY user_id;

-- Solution: Use any() or other aggregate function
SELECT 
    user_id,
    any(name) as name,
    count() as events
FROM users
GROUP BY user_id;

3. Fix subquery column references

-- Error: column 'user_id' from outer query used incorrectly
SELECT 
    user_id,
    (SELECT max(event_time) FROM events WHERE user_id = users.user_id) as last_event
FROM users;

-- Solution: Use proper correlation
SELECT 
    u.user_id,
    (SELECT max(event_time) FROM events e WHERE e.user_id = u.user_id) as last_event
FROM users u;

4. Qualify column names with table aliases in JOINs

-- Error: Ambiguous column reference
SELECT 
    id,
    name
FROM users
JOIN orders USING (id);

-- Solution: Use table aliases or qualify columns
SELECT 
    u.id as user_id,
    u.name,
    o.id as order_id
FROM users u
JOIN orders o ON u.id = o.user_id;

5. Move window function logic to subquery or CTE

-- Error: Can't use window function in WHERE
SELECT 
    user_id,
    row_number() OVER (PARTITION BY user_id ORDER BY event_time) as rn
FROM events
WHERE rn = 1;

-- Solution: Use subquery or CTE
WITH ranked AS (
    SELECT 
        user_id,
        event_time,
        row_number() OVER (PARTITION BY user_id ORDER BY event_time) as rn
    FROM events
)
SELECT user_id, event_time
FROM ranked
WHERE rn = 1;

6. Fix ARRAY JOIN scope issues

-- Error: Incorrect array column reference
SELECT 
    user_id,
    tag
FROM users
ARRAY JOIN tags as tag
WHERE user_id IN (SELECT user_id FROM users WHERE tag = 'premium');

-- Solution: Restructure to ensure proper scope
WITH tagged_users AS (
    SELECT DISTINCT user_id
    FROM users
    ARRAY JOIN tags as tag
    WHERE tag = 'premium'
)
SELECT 
    u.user_id,
    t.tag
FROM users u
ARRAY JOIN u.tags as t
WHERE u.user_id IN (SELECT user_id FROM tagged_users);

7. Use proper aggregation in HAVING clauses

-- Error: name not in GROUP BY or aggregate
SELECT 
    category,
    count() as cnt
FROM products
GROUP BY category
HAVING name LIKE '%special%';

-- Solution: Use aggregate function or move to WHERE
SELECT 
    category,
    count() as cnt
FROM products
WHERE name LIKE '%special%'
GROUP BY category;

Prevention tips​

  1. Always aggregate non-GROUP BY columns: When using GROUP BY, ensure every column in SELECT is either in the GROUP BY clause or wrapped in an aggregate function (count, sum, any, etc.)
  2. Use explicit table aliases in JOINs: Always qualify column names with table aliases when working with multiple tables to avoid ambiguity and improve query clarity
  3. Understand query evaluation order: Remember that SQL evaluates in order: FROM → WHERE → GROUP BY → HAVING → SELECT → ORDER BY → LIMIT. Use this to understand where columns are available
  4. Test complex queries incrementally: Build complex queries step by step, testing each level of nesting or each JOIN separately to identify where column scope issues arise
  5. Use CTEs for complex window functions: When using window functions, consider using CTEs (WITH clauses) to separate the window function evaluation from filtering operations
  6. Enable strict mode settings: Use any_join_distinct_right_table_keys = 1 and other strict settings during development to catch column reference issues early
  7. Validate column existence: Before running complex queries in production, verify that all referenced columns exist in their respective tables and are accessible in the query context