🛀 SQL CAST / CONVERT – Syntax, Differences & Use Cases

🧲 Introduction – Why Type Conversion Is Essential in SQL

Data types matter. Whether you’re comparing numbers and strings, working with dates, or combining data from multiple sources—you often need to convert values from one type to another.

That’s where CAST() and CONVERT() come in. These SQL functions allow you to explicitly change data types within queries to avoid errors, improve compatibility, and ensure consistent formatting.

🎯 In this guide, you’ll learn:

• The syntax and usage of CAST() and CONVERT()
• Key differences across MySQL, PostgreSQL, and SQL Server
• Real-world examples of casting between text, numbers, and dates
• Best practices and common pitfalls

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🔀 1. What Is SQL Type Conversion?

Type conversion (aka casting) is the process of changing a value’s data type—for example, from VARCHAR to INT, or from DATE to STRING.

There are two forms:

• Implicit – SQL automatically converts types when needed.
• Explicit – You use CAST() or CONVERT() to ensure correctness.

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🛠️ 2. SQL CAST() Syntax & Examples

✅ Basic Syntax

CAST(expression AS target_data_type)

✅ Examples

-- Convert string to integer
SELECT CAST('123' AS INT); -- → 123

-- Convert decimal to integer
SELECT CAST(123.45 AS INT); -- → 123

-- Convert integer to string
SELECT CAST(456 AS VARCHAR); -- → '456'

-- Convert date to string (SQL Server)
SELECT CAST(GETDATE() AS VARCHAR); -- → '2024-05-16 ...'

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🔧 3. SQL CONVERT() Syntax & Examples (SQL Server Only)

CONVERT(target_data_type, expression [, style])

✅ Examples

-- Convert string to date with style
SELECT CONVERT(DATE, '2024-05-16', 120); -- ISO format

-- Convert float to integer
SELECT CONVERT(INT, 9.99); -- → 9

-- Convert datetime to string (style 103 = dd/mm/yyyy)
SELECT CONVERT(VARCHAR, GETDATE(), 103); -- → '16/05/2024'

📘 style controls date/time formatting (SQL Server-specific).

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🧭 4. Platform Differences: CAST vs CONVERT

Feature	MySQL	PostgreSQL	SQL Server
CAST()	✅ Yes	✅ Yes	✅ Yes
CONVERT()	✅ Yes (numeric/date)	❌ No (reserved)	✅ Yes
Style argument	❌ No	❌ No	✅ Yes (CONVERT)

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🎯 5. Real-World Use Cases

Use Case	Example
Compare string to number	CAST(col AS INT)
Format a date for display	CAST(date AS VARCHAR)
Truncate decimals for reporting	CAST(price AS INT)
Store calculated value as string	CAST(SUM(total) AS CHAR) (MySQL)
Format API JSON response in SQL	CAST(col AS TEXT) (PostgreSQL/JSON)

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⚠️ 6. Common Pitfalls & Best Practices

⚠️ Mistake	✅ Best Practice
Relying on implicit type conversion	Always use CAST() for clarity
Using CONVERT() in PostgreSQL	Use only CAST()—CONVERT is not valid
Losing precision on decimal → int	Round or format before cast if needed
Wrong date format in CONVERT style	Reference correct style codes (SQL Server)

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📌 Summary – Recap & Next Steps

CAST() and CONVERT() are powerful tools for transforming data types in SQL. Use them to clean, compare, and display data properly, especially when mixing inputs across different types or preparing data for APIs and reports.

🔍 Key Takeaways:

• Use CAST() for portable, ANSI-standard conversions.
• Use CONVERT() in SQL Server for date styles and advanced formats.
• Always cast explicitly in JOINs, WHERE, and CASE statements.

⚙️ Real-World Relevance:
Data type mismatches are a top cause of SQL bugs. Casting ensures accuracy, compatibility, and readability in all environments.

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❓ FAQ – SQL CAST / CONVERT

❓ When should I use CAST vs CONVERT?

✅ Use CAST() for standard SQL across databases. Use CONVERT() in SQL Server when you need specific formatting styles.

❓ Can I cast strings to dates?

✅ Yes—if the string format is valid. Use CAST('2024-05-16' AS DATE) or CONVERT(DATE, ...) with style codes.

❓ What happens if a CAST fails?

❌ It typically throws an error unless you’re using TRY_CAST() or TRY_CONVERT() (SQL Server).

❓ Is CAST() faster than CONVERT()?

⚖️ Performance is comparable. CAST() is preferred for cross-platform compatibility.

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