It’s annoying when code errors occur, and it can be extremely difficult to spot and correct your errors when you’re still learning SQL. Code errors are frequent and annoying. Additionally, it might be very difficult to identify and correct your errors when you’re still learning SQL. We’ll outline eight solutions to common SQL code errors in this article.
In this article, we’ll go through some tips that a beginner SQL user can use to steer clear of numerous common mistakes. Any database environment can use these recommendations. We’ll display various error warnings as we go along. Your error messages may not be written in the same way, but don’t worry—while each database engine has its own variations, the meaning is always the same.
So we won’t be concentrating on error messages. Instead, we’ll identify the root of the issue (such as failing to utilize a parenthesis or quotation mark). In this manner, you’ll discover how to avoid the problem altogether. Let’s start!
Also Read: Most Important SQL Queries for Beginners
1. Use the Correct SQL Statements
Make sure you are using the correct SQL statements for the task at hand. For example, if you want to retrieve data from a table, use the
SELECT statement. If you want to update data, use the
UPDATE statement. Using the wrong statement can lead to errors.
2. First Place Open and Close Parentheses
Eliminating unbalanced brackets, quotations, double quotes or square brackets requires keeping in mind the ending character. Best practices advise typing the opening and closing characters first, followed by the text that appears inside. When brackets are not balanced like in the example below, an error occurs:
SELECT firstname, country FROM employee WHERE salary >= (select AVG(salary) from employee ;Code language: SQL (Structured Query Language) (sql)
ERROR: syntax error at or near ";" LINE 2: WHERE salary >= (select AVG(salary) from employee ;Code language: SQL (Structured Query Language) (sql)
3. Use Proper Formatting
By correctly formatting your SQL queries, you can make them simpler to read and comprehend. Make the code more understandable by using indentation, and use line breaks to divide the query into its component components. This will make it easier for you to see mistakes. Take a look at the following example:
SELECT id, FirstName,c.nAme FROM Person p left JOIN cities AS c on c.id=p.cityid;Code language: SQL (Structured Query Language) (sql)
There are no formatting guidelines used in the writing of the above SQL query. Let’s compare that to the structured query that uses the same code as the one below:
SELECT p.PersonId, p.FirstName, c.Name FROM Person AS p LEFT JOIN City AS c ON p.CityId = c.CityId;Code language: SQL (Structured Query Language) (sql)
As you can see the difference! This is more readable and will reduce the chances of errors. So proper formatting is essential for both beginners and professionals.
4. Don’t End a Column or Table Sequence with a Comma
This mistake happens a lot. In SQL, commas serve as a separator. There shouldn’t be any commas between FROM and the first table name or after the final table name. The same rule applies to column definition: avoid adding an extra comma after the final column name when creating a table.
SELECT * FROM sales, department,Code language: SQL (Structured Query Language) (sql)
ERROR: syntax error at or near "," LINE 1: SELECT * FROM sales, departmentCode language: SQL (Structured Query Language) (sql)
5. Proper Commenting
Keep your code comments to a minimum. Of course, there are situations in which comments are required, but it is often preferable to utilize multiple-line comments, which are denoted by the letters /* opening and */ closing characters. It is advised to write this type of comment at the start of a new line, instead of beginning on a line that contains code that is executed. For example:
SELECT p.PersonId, p.FirstName, /* Name column is the name of the city: */ p.Name, FROM Person AS p WHERE p.Name = 'New York';Code language: SQL (Structured Query Language) (sql)
One-line comments can also be added to SQL code. A double hyphen (–) at the start of the comment text specifies this kind of comment. All text after these characters is treated as a comment. For example,
SELECT * FROM Customers -- WHERE City='New York';Code language: SQL (Structured Query Language) (sql)
6. Double Check Column and Table Names
When typing table or column names, be extremely careful. If at all possible, attempt to copy and paste the name of a command that you are sure is accurate—ideally, one that has previously been correctly executed. Even if you believe typing is faster, best practices advise copying and pasting names. SQL is case-sensitive, so be sure to use the correct case when referring to table and column names.
Always check the column name for spelling errors, confirm that the table in the FROM contains this column, and confirm that the table is mentioned in the FROM. For example:
SELECT lastname, firstname FROM employeesCode language: SQL (Structured Query Language) (sql)
ERROR: table "employees" does not exist LINE 1: SELECT lastname, firstname FROM employeesCode language: SQL (Structured Query Language) (sql)
Using a table alias or table name as a column prefix is another recommended practice. When you have two or more tables in the FROM clause, this becomes much more important. Referring to two columns in distinct tables with the same names can result in the following error:
SELECT firstname, lastname FROM department, employee WHERE dept_id = dept_idCode language: SQL (Structured Query Language) (sql)
ERROR: column reference "dept_id" is ambiguous LINE 3: WHERE dept_id = dept_idCode language: SQL (Structured Query Language) (sql)
To fix these errors, we will add the table name before the column name like this,
SELECT lastname, name FROM department, employee WHERE department.dept_id = employee.dept_idCode language: SQL (Structured Query Language) (sql)
7. Check Data Types while Comparing
Ensure that the data types you are using match the data types of the columns in your table. If the data types do not match, you may encounter errors. Make sure that both data types are compatible with the comparison operator and with each other when writing comparison conditions in the WHERE clause. If it’s not possible, you might need to cast one of the data types. As a general guideline, compare numerical values to numerical values, character strings to character strings, etc.
For example, CHAR string and integer comparisons in the following SQL code result in a data mismatch error:
SELECT lastname, salary FROM sales WHERE user_id = firstnameCode language: SQL (Structured Query Language) (sql)
ERROR: operator does not exist: integer = character varying LINE 3: WHERE user_id = firstnameCode language: SQL (Structured Query Language) (sql)
8. Use Sub-Queries
If you’re having trouble with a complex query, try breaking it down into smaller sub-queries. This will help you to better understand the logic of your query and to identify any errors. Subqueries make it simpler to get the data you require from your database by dividing large searches into smaller, more manageable pieces.
Here, take a look at below multiple-row sub-query example. Let’s say we have a table of Orders and a table of Items. The names of every item that has been ordered more than 5 times must be retrieved. To get the item names, we can use a multiple-row subquery:
SELECT item_name FROM Items WHERE Item_id IN (SELECT item_id FROM orders GROUP BY item_id HAVING COUNT(*) > 5);Code language: SQL (Structured Query Language) (sql)
The 8 tips we have seen in this tutorial are 1. Use correct SQL statements, 2. First Place Open and Close Parentheses, 3. 3. Use Proper Formatting, 4. Don’t End a Column or Table Sequence with a Comma, 5. Proper Commenting, 6. Double Check Column and Table Names, 7. Check Data Types while Comparing, 8. Use Sub-Queries. These tips will surely help you fix and optimize your SQL queries. Try it yourself to see if it benefits you while writing queries.