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SQL Syntax Checker Examples

Reference for sql-syntax-checker · 643 lines

Programming / SQLv1.0.0Updated OCT 20 20261 reference1 script

Common syntax errors with explanations and corrections.

Structural Errors

Missing FROM Clause

Error:

sql
SELECT id, name WHERE status = 'active';

Fixed:

sql
SELECT id, name FROM users WHERE status = 'active';

Explanation: SELECT requires a FROM clause to specify the source table.

Wrong Clause Order

Error:

sql
SELECT * FROM users ORDER BY name WHERE status = 'active';

Fixed:

sql
SELECT * FROM users WHERE status = 'active' ORDER BY name;

Explanation: WHERE must come before ORDER BY. Correct order: FROM -> WHERE -> GROUP BY -> HAVING -> ORDER BY -> LIMIT.

Missing Semicolon (Multi-Statement)

Error:

sql
SELECT * FROM users
SELECT * FROM orders

Fixed:

sql
SELECT * FROM users;
SELECT * FROM orders;

Explanation: When running multiple statements, each must be terminated with a semicolon.

Quote and Literal Errors

Wrong String Quotes

Error (Standard SQL):

sql
SELECT * FROM users WHERE name = "John";

Fixed:

sql
SELECT * FROM users WHERE name = 'John';

Explanation: Standard SQL uses single quotes for string literals. Double quotes are for identifiers (or backticks in MySQL).

Unescaped Quote in String

Error:

sql
SELECT * FROM posts WHERE title = 'It's a test';

Fixed:

sql
SELECT * FROM posts WHERE title = 'It''s a test';

Or with escape (PostgreSQL, MySQL):

sql
SELECT * FROM posts WHERE title = 'It\'s a test';

Explanation: Single quotes inside strings must be escaped by doubling them.

Missing Quotes on String

Error:

sql
SELECT * FROM users WHERE status = active;

Fixed:

sql
SELECT * FROM users WHERE status = 'active';

Explanation: String values must be enclosed in quotes; otherwise they're interpreted as column names.

Column and Reference Errors

Misspelled Column Name

Error:

sql
SELECT id, nmae, email FROM users;

Fixed:

sql
SELECT id, name, email FROM users;

Explanation: Column names must match exactly (case-sensitivity depends on dialect and configuration).

Missing Comma in Column List

Error:

sql
SELECT 
    id
    name
    email
FROM users;

Fixed:

sql
SELECT 
    id,
    name,
    email
FROM users;

Explanation: Columns in SELECT must be separated by commas.

Extra Comma at End

Error:

sql
SELECT id, name, email, FROM users;

Fixed:

sql
SELECT id, name, email FROM users;

Explanation: Trailing comma before FROM is not allowed.

Ambiguous Column in JOIN

Error:

sql
SELECT id, name, order_date 
FROM users u 
JOIN orders o ON u.id = o.user_id;

Fixed:

sql
SELECT u.id, u.name, o.order_date 
FROM users u 
JOIN orders o ON u.id = o.user_id;

Explanation: When joining tables, columns that exist in multiple tables must be qualified with table alias.

JOIN Errors

Missing ON Condition

Error:

sql
SELECT * FROM users u JOIN orders o;

Fixed:

sql
SELECT * FROM users u JOIN orders o ON u.id = o.user_id;

Explanation: INNER/LEFT/RIGHT JOIN requires an ON clause specifying the join condition.

CROSS JOIN with ON

Error:

sql
SELECT * FROM users CROSS JOIN roles ON users.role_id = roles.id;

Fixed (if you want filtered join):

sql
SELECT * FROM users JOIN roles ON users.role_id = roles.id;

Fixed (if you want cartesian product):

sql
SELECT * FROM users CROSS JOIN roles;

Explanation: CROSS JOIN produces cartesian product and doesn't use ON clause.

Using WHERE Instead of ON

Technically valid but poor practice:

sql
SELECT * FROM users u, orders o WHERE u.id = o.user_id;

Better:

sql
SELECT * FROM users u JOIN orders o ON u.id = o.user_id;

Explanation: Explicit JOIN syntax is clearer and prevents accidental cartesian products.

GROUP BY Errors

Non-Aggregated Column Not in GROUP BY

Error:

sql
SELECT department, employee_name, COUNT(*)
FROM employees
GROUP BY department;

Fixed (aggregate the column):

sql
SELECT department, STRING_AGG(employee_name, ', '), COUNT(*)
FROM employees
GROUP BY department;

Or (add to GROUP BY):

sql
SELECT department, employee_name, COUNT(*)
FROM employees
GROUP BY department, employee_name;

Explanation: All columns in SELECT must either be in GROUP BY or be aggregated.

HAVING Without GROUP BY

Warning (valid but suspicious):

sql
SELECT * FROM orders HAVING total > 100;

Probably meant:

sql
SELECT * FROM orders WHERE total > 100;

Or:

sql
SELECT customer_id, SUM(total) 
FROM orders 
GROUP BY customer_id 
HAVING SUM(total) > 100;

Explanation: HAVING is for filtering groups; use WHERE for row-level filtering.

Using Column Alias in HAVING

Error (most dialects):

sql
SELECT department, COUNT(*) AS emp_count
FROM employees
GROUP BY department
HAVING emp_count > 5;

Fixed:

sql
SELECT department, COUNT(*) AS emp_count
FROM employees
GROUP BY department
HAVING COUNT(*) > 5;

Explanation: Column aliases defined in SELECT are not available in HAVING (except MySQL).

Subquery Errors

Subquery Returns Multiple Rows with = Operator

Error:

sql
SELECT * FROM products 
WHERE category_id = (SELECT id FROM categories WHERE active = true);

Fixed (if expecting multiple):

sql
SELECT * FROM products 
WHERE category_id IN (SELECT id FROM categories WHERE active = true);

Fixed (if expecting one):

sql
SELECT * FROM products 
WHERE category_id = (SELECT id FROM categories WHERE active = true LIMIT 1);

Explanation: When using = with subquery, the subquery must return exactly one value.

Subquery Column Not Accessible

Error:

sql
SELECT * FROM users 
WHERE id IN (SELECT user_id, amount FROM orders WHERE amount > 100);

Fixed:

sql
SELECT * FROM users 
WHERE id IN (SELECT user_id FROM orders WHERE amount > 100);

Explanation: Subquery used with IN must return exactly one column.

Correlated Subquery Reference Error

Error:

sql
SELECT * FROM orders o 
WHERE total > (SELECT AVG(total) FROM orders WHERE customer_id = c.id);

Fixed:

sql
SELECT * FROM orders o 
WHERE total > (SELECT AVG(total) FROM orders WHERE customer_id = o.customer_id);

Explanation: Correlated subqueries must reference columns from the outer query using correct alias.

NULL Handling Errors

Using = NULL Instead of IS NULL

Error:

sql
SELECT * FROM users WHERE deleted_at = NULL;

Fixed:

sql
SELECT * FROM users WHERE deleted_at IS NULL;

Explanation: NULL comparisons must use IS NULL or IS NOT NULL; = NULL always returns unknown.

NOT IN with NULL Values

Problematic:

sql
SELECT * FROM products 
WHERE id NOT IN (SELECT product_id FROM discontinued);

If discontinued.product_id contains NULL, this returns no rows.

Fixed:

sql
SELECT * FROM products 
WHERE id NOT IN (SELECT product_id FROM discontinued WHERE product_id IS NOT NULL);

Or better:

sql
SELECT * FROM products p
WHERE NOT EXISTS (SELECT 1 FROM discontinued d WHERE d.product_id = p.id);

Type Mismatch Errors

Comparing String to Number

Error:

sql
SELECT * FROM users WHERE id = 'abc';

Fixed (if id is numeric):

sql
SELECT * FROM users WHERE id = 123;

Fixed (if looking for string):

sql
SELECT * FROM users WHERE username = 'abc';

Date Format Issues

Error (ambiguous):

sql
SELECT * FROM orders WHERE order_date = '01/02/2024';

Fixed (use ISO format):

sql
SELECT * FROM orders WHERE order_date = '2024-01-02';

Explanation: ISO 8601 format (YYYY-MM-DD) is unambiguous and works across all dialects.

Boolean Comparison

Error (Oracle has no boolean):

sql
SELECT * FROM users WHERE is_active = TRUE;

Fixed (Oracle):

sql
SELECT * FROM users WHERE is_active = 1;

Fixed (PostgreSQL, most others):

sql
SELECT * FROM users WHERE is_active = true;

Or simply:

sql
SELECT * FROM users WHERE is_active;

Pagination Errors

OFFSET Without LIMIT

Error (PostgreSQL):

sql
SELECT * FROM products OFFSET 10;

Fixed:

sql
SELECT * FROM products LIMIT 100 OFFSET 10;

MySQL LIMIT Syntax Mix-up

Error:

sql
SELECT * FROM products LIMIT 10, 20 OFFSET 0;

Fixed (choose one syntax):

sql
SELECT * FROM products LIMIT 20 OFFSET 10;

Or:

sql
SELECT * FROM products LIMIT 10, 20;

Explanation: MySQL's LIMIT offset, count and LIMIT count OFFSET offset shouldn't be mixed.

CTE Errors

Missing CTE Name

Error:

sql
WITH AS (
    SELECT * FROM users WHERE status = 'active'
)
SELECT * FROM active_users;

Fixed:

sql
WITH active_users AS (
    SELECT * FROM users WHERE status = 'active'
)
SELECT * FROM active_users;

CTE Not Used

Warning:

sql
WITH active_users AS (
    SELECT * FROM users WHERE status = 'active'
)
SELECT * FROM users;

Fixed:

sql
WITH active_users AS (
    SELECT * FROM users WHERE status = 'active'
)
SELECT * FROM active_users;

Recursive CTE Missing UNION

Error:

sql
WITH RECURSIVE org_tree AS (
    SELECT id, name, manager_id FROM employees WHERE manager_id IS NULL
)
SELECT * FROM org_tree;

Fixed:

sql
WITH RECURSIVE org_tree AS (
    SELECT id, name, manager_id, 1 AS level 
    FROM employees 
    WHERE manager_id IS NULL
    
    UNION ALL
    
    SELECT e.id, e.name, e.manager_id, ot.level + 1
    FROM employees e
    JOIN org_tree ot ON e.manager_id = ot.id
)
SELECT * FROM org_tree;

INSERT/UPDATE/DELETE Errors

INSERT Column Count Mismatch

Error:

sql
INSERT INTO users (id, name, email)
VALUES (1, 'John');

Fixed:

sql
INSERT INTO users (id, name, email)
VALUES (1, 'John', 'john@example.com');

UPDATE Without WHERE

Warning (affects all rows):

sql
UPDATE users SET status = 'inactive';

Probably meant:

sql
UPDATE users SET status = 'inactive' WHERE last_login < '2023-01-01';

DELETE Without WHERE

Warning (deletes all rows):

sql
DELETE FROM users;

Probably meant:

sql
DELETE FROM users WHERE status = 'deleted';

Or:

sql
TRUNCATE TABLE users;

SET Using Wrong Operator

Error:

sql
UPDATE users SET status == 'active' WHERE id = 1;

Fixed:

sql
UPDATE users SET status = 'active' WHERE id = 1;

Explanation: Assignment in SET uses single equals =, not comparison ==.

Window Function Errors

Window Function in WHERE

Error:

sql
SELECT * FROM employees 
WHERE ROW_NUMBER() OVER (ORDER BY salary DESC) <= 10;

Fixed:

sql
SELECT * FROM (
    SELECT *, ROW_NUMBER() OVER (ORDER BY salary DESC) AS rn
    FROM employees
) ranked
WHERE rn <= 10;

Explanation: Window functions cannot be used in WHERE; use a subquery or CTE.

Missing OVER Clause

Error:

sql
SELECT id, name, ROW_NUMBER() FROM employees;

Fixed:

sql
SELECT id, name, ROW_NUMBER() OVER (ORDER BY id) FROM employees;

Explanation: Window functions require an OVER clause.

Invalid Frame Specification

Error:

sql
SELECT id, SUM(amount) OVER (ORDER BY date ROWS UNBOUNDED) 
FROM transactions;

Fixed:

sql
SELECT id, SUM(amount) OVER (ORDER BY date ROWS UNBOUNDED PRECEDING) 
FROM transactions;

Explanation: Frame specification needs PRECEDING or FOLLOWING keyword.

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