Estimated reading time: 3 minutes
SQL ERRORS
And how to resolve them
By Lovee Kumar
A quick guide on common SQL errors
In SQL, Errors can often occur due to various reasons. Here’s a list of some common ones and how to resolve them.
Duplicate Values
Happens when you try to insert data with a primary key that already exists.
Error:
INSERT INTO table1 (id, name) VALUES (1, 'John');If ID is a primary key and there’s already a row with ID 1, this will fail. You need to insert a unique ID.
Solution:
INSERT INTO table1 (id, name) VALUES (2, 'John');Null Values
Occurs when you try to perform operations on null values.
Error:
SELECT column1 + column2 FROM table1;If column1 or column2 has a null value, it’ll fail. You need to handle nulls, perhaps with COALESCE.
Solution:
SELECT COALESCE(column1, 0) + COALESCE(column2, 0) FROM table1;Data Type Mismatch
Occurs when you try to insert or compare data of different data types.
Error:
SELECT column1 FROM table1 WHERE column2 = '5';Here, column2 is presumed to be an integer. So, the ‘5’ should not be in quotes.
Solution:
SELECT column1 FROM table1 WHERE column2 = 5;Case Sensitivity
SQL is case sensitive. If you incorrectly use uppercase or lowercase letters, it may lead to errors.
Error:
SELECT Column1 FROM table1;If the column name is actually ‘column1’ and not ‘Column1’, this will fail. SQL is case sensitive.
Solution:
SELECT column1 FROM table1;Incorrect Use Of Joins
Occurs when you use functions incorrectly or with the wrong data types.
Error:
SELECT * FROM table1 JOIN table2;You need to provide the join condition using the ON clause.
Solution:
SELECT * FROM table1 JOIN table2 ON table1.id = table2.id;Inconsistent Naming Conventions
Occurs when you use inconsistent naming conventions for tables, columns, and other database objects.
Error:
SELECT columnOne FROM table1;
SELECT column1 FROM table1;Stick with a single naming convention to avoid confusion. Here, columnOne and column1 are presumably the same, but named differently.
Solution:
SELECT column1 FROM table1;Ambiguous Column Name
Happens when you reference a column name that exists in multiple tables.
Error:
SELECT id FROM table1 JOIN table2 ON table1.id = table2.id;If both table1 and table2 have a column named id, this will fail. You need to specify which table’s id to select.
Solution:
SELECT table1.id FROM table1 JOIN table2 ON table1.id = table2.id;Improper Use Of Function
Occurs when you use functions incorrectly or with the wrong data types.
Improper Use Of Function
Occurs when you use functions incorrectly or with the wrong data types.
Error:
SELECT SUBSTRING(column1, '1', '5') FROM table1;The second and third parameters of SUBSTRING should be integers.
Solution:
SELECT SUBSTRING(column1, 1, 5) FROM table1;Incorrect Use of Quotes
Occurs when you use the wrong type of quotes or forget to enclose text values in quotes.
Error:
SELECT * FROM table1 WHERE name = "John";In SQL, string literals should be enclosed in single quotes.
Solution:
SELECT * FROM table1 WHERE name = 'John';Incorrect Use Of Group By
Occurs when you use the GROUP BY clause incorrectly or with the wrong syntax.
Error:
SELECT department_id, COUNT(*) FROM employees GROUP BY name;Use the correct columns for the GROUP BY clause.
Solution:
SELECT department_id, COUNT(*) FROM employees GROUP BY department_id;Important Notes
- The exact behavior of these SQL statements may depend on the specific SQL variant and database system you are using.
- Don’t be afraid to make errors – nothing teaches better than making errors and debugging.
PRACTICE MAKES PERFECT
Follow For Lovee Kumar