Writing Parameterized Queries in SQL
💡 Concept Name
Parameterized Queries – SQL statements that use placeholders instead of directly embedding user input into queries.
📘 Quick Intro
Parameterized queries ensure safe database access by binding values instead of string-concatenating input. They help prevent SQL injection by separating code from data. Most programming languages support this via placeholders (e.g., @param, ?, or :). They also help optimize performance through execution plan reuse.
🧠 Analogy / Short Story
Imagine placing an order by filling in blanks on a standard form instead of writing your own sentence. The form defines the structure (query), and your input (parameter) is safely inserted in the right place. Without forms, anyone could sneak in extra words and change the request entirely—just like SQL injection. Parameterized queries are your security-locked order forms.
🔧 Technical Explanation
- ✅ Use placeholders (e.g.,
@UserId) in SQL instead of string-concatenated values. - 🧱 Prepared statements ensure values are escaped, avoiding SQL injection.
- 🔁 Reused execution plans improve database performance and reduce parsing overhead.
- 🔒 Application frameworks like ADO.NET, JDBC, and Python's
sqlite3support parameters natively. - 🧪 Always validate and bind user input rather than injecting it into query strings.
🎯 Purpose & Use Case
- ✅ Prevent SQL injection attacks.
- ✅ Simplify dynamic queries without risking malformed SQL.
- ✅ Improve execution plan caching and reuse.
- ✅ Enhance code readability and separation of logic and data.
💻 Real Code Example
-- SQL Server (T-SQL)
SELECT * FROM Users WHERE UserId = @UserId;
-- C# with SqlCommand
cmd.CommandText = "SELECT * FROM Users WHERE UserId = @id";
cmd.Parameters.AddWithValue("@id", 123);
-- MySQL with Python
cursor.execute("SELECT * FROM Users WHERE UserId = %s", (123,))
❓ Interview Q&A
Q1: What are parameterized queries?
A: SQL queries where input values are supplied as parameters, not directly inserted into the query string.
Q2: Why are parameterized queries more secure?
A: They prevent SQL injection by separating data from query logic and ensuring input is properly escaped.
Q3: Are parameterized queries faster than dynamic SQL?
A: Yes, they allow execution plan reuse and require less parsing by the database.
Q4: How do you write a parameterized query in C#?
A: Use placeholders (e.g., @name) and bind them using cmd.Parameters.AddWithValue().
Q5: Can you use parameters in INSERT and UPDATE?
A: Yes, parameterized queries are supported in all SQL operations including INSERT, UPDATE, and DELETE.
Q6: What happens if you use string concatenation instead?
A: You risk SQL injection and may lose performance benefits from execution plan reuse.
Q7: How does a prepared statement relate to parameterized queries?
A: It’s a form of parameterized query where the database parses and compiles the query in advance.
Q8: Can all databases use parameterized queries?
A: Yes, most modern SQL databases and their APIs support it, including SQL Server, PostgreSQL, MySQL, and SQLite.
Q9: What is a bind variable?
A: It’s a placeholder in a query replaced with a value at runtime, used in parameterized queries.
Q10: Should values ever be embedded into SQL strings directly?
A: No. Always use parameters to avoid security and performance issues.
📝 MCQs
Q1. What is the purpose of a parameterized query?
- To sort data
- To combine tables
- To separate data from SQL logic
- To export reports
Q2. Which statement uses a parameter?
- SELECT * FROM Users WHERE id = 123
- SELECT * FROM Users
- SELECT * FROM Users WHERE id = @id
- SELECT id FROM Users
Q3. What does parameterized query prevent?
- Joins
- SQL injection
- Duplicate rows
- Primary keys
Q4. Which C# method binds a parameter?
- AddTable
- AddField
- SetId
- AddWithValue
Q5. What's a bind variable?
- Column name
- Temporary table
- Placeholder replaced with runtime value
- Data type
Q6. What does a prepared statement do?
- Deletes rows
- Pre-compiles query with placeholders
- Updates indexes
- Runs scripts
Q7. Why reuse execution plans?
- Better formatting
- Improves performance
- Less storage
- Easier joins
Q8. In Python's MySQL, which symbol is used for parameters?
- @
- :
- %
- #
Q9. What happens when parameters are embedded via string concatenation?
- Query runs faster
- Query becomes read-only
- Query becomes vulnerable to injection
- Database is locked
Q10. Which operation benefits from parameters?
- SELECT
- INSERT
- UPDATE
- All of the above
💡 Bonus Insight
Always use parameterized queries, even in internal systems. Even trusted input can be malformed or manipulated. Tools like ORMs (Entity Framework, Sequelize, Django ORM) use parameters under the hood—leverage them wisely. Secure code is performant code.
📄 PDF Download
Need a handy summary for your notes? Download this topic as a PDF!