🧬 SQL Data Types – Full Guide to SQL Types & Syntax

🧲 Introduction – Why SQL Data Types Matter

When creating a table in SQL, one of the most crucial decisions is choosing the correct data type for each column. Why? Because the data type defines what kind of data (numeric, textual, date/time, etc.) can be stored and how it’s stored and processed.

A mismatched data type can result in storage inefficiencies, runtime errors, or even data loss. Whether you’re designing a schema or querying complex records, understanding SQL data types ensures database integrity and performance.

🎯 In this guide, you’ll learn:

• Core SQL data type categories
• Dialect differences (MySQL, SQL Server, PostgreSQL)
• Real-world usage examples
• Common mistakes and best practices

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🔑 1. SQL Data Type Categories

SQL data types are grouped into several broad categories:

Category	Description	Examples
📊 Numeric	Stores numeric values (integers, decimals)	INT, DECIMAL, FLOAT
🔤 String	Stores characters or text	CHAR, VARCHAR, TEXT
⏱️ Date/Time	Stores dates and times	DATE, TIME, TIMESTAMP
🧬 Binary	Stores binary data like images or files	BLOB, BINARY, VARBINARY
🔒 Boolean/Bit	Stores logical values (true/false, 0/1)	BOOLEAN, BIT
🧩 Special	Others depending on dialect (e.g., ENUM, UUID)	ENUM, JSON, XML, UUID

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📊 2. Numeric Data Types

Used for calculations, counts, and IDs.

CREATE TABLE accounts (
  id INT,
  balance DECIMAL(10, 2),
  interest_rate FLOAT
);

✅ Explanation:

• INT – Stores whole numbers.
• DECIMAL(10,2) – Fixed-point number with precision for money.
• FLOAT – Approximate numeric value with decimals.

💡 Tip: Use DECIMAL for financial applications to avoid rounding issues.

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🔤 3. String Data Types

Handle names, addresses, notes, and text-heavy fields.

CREATE TABLE users (
  username VARCHAR(30),
  bio TEXT,
  country CHAR(2)
);

✅ Explanation:

• VARCHAR(n) – Variable-length string.
• TEXT – Large unbounded text.
• CHAR(n) – Fixed-length string (ideal for short codes like country codes).

⚠️ Warning: Overuse of TEXT can lead to performance issues.

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⏱️ 4. Date and Time Data Types

Used for tracking time-related records.

CREATE TABLE events (
  event_name VARCHAR(100),
  start_time TIMESTAMP,
  end_date DATE
);

✅ Explanation:

• DATE – Stores date only.
• TIME – Stores time only.
• TIMESTAMP – Stores full date and time.

📘 Best Practice: Use TIMESTAMP WITH TIME ZONE (if supported) for global apps.

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🧬 5. Binary Data Types

For storing images, files, or encrypted data.

CREATE TABLE files (
  file_name VARCHAR(100),
  data BLOB
);

✅ Explanation:

• BLOB – Binary Large Object (used for raw binary files).
• BINARY(n) – Fixed length.
• VARBINARY(n) – Variable length.

⚠️ Caution: Binary fields increase database size and can affect query performance.

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🔒 6. Boolean & Bit Data Types

Store binary logical values.

CREATE TABLE settings (
  feature_enabled BOOLEAN,
  flag BIT
);

✅ Explanation:

• BOOLEAN – Logical true/false (TINYINT in MySQL).
• BIT – Binary digit, commonly used for flags.

💡 Tip: Use constraints to enforce valid binary values (0 or 1).

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🧩 7. Special and Dialect-Specific Data Types

SQL Dialect	Special Types
MySQL	ENUM, SET, YEAR
PostgreSQL	UUID, JSONB, ARRAY, TSVECTOR
SQL Server	UNIQUEIDENTIFIER, XML, MONEY

✅ Examples:

-- PostgreSQL UUID
id UUID DEFAULT gen_random_uuid();

-- MySQL ENUM
status ENUM('active', 'inactive', 'pending');

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⚙️ 8. SQL Data Type Comparison Table

Data Type	MySQL	PostgreSQL	SQL Server
Integer	INT	INTEGER	INT
Decimal	DECIMAL	NUMERIC	DECIMAL
Text	TEXT	TEXT	TEXT / NVARCHAR(MAX)
Date/Time	DATETIME	TIMESTAMP	DATETIME2
Boolean	TINYINT(1)	BOOLEAN	BIT
UUID	No native	UUID	UNIQUEIDENTIFIER
JSON	JSON	JSONB	NVARCHAR (manual)

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📘 Best Practices for Choosing Data Types

💡 Do This	❌ Avoid This
Use VARCHAR(n) for variable strings	Using TEXT for short text
Use DECIMAL for money	Using FLOAT for currency
Specify precision and scale where needed	Leaving size unspecified (e.g. VARCHAR)
Normalize date/time formats	Mixing DATE and CHAR for dates

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📌 Summary – Recap & Next Steps

Understanding SQL data types is critical for schema design, query accuracy, and performance optimization.

🔍 Key Takeaways:

• Choose data types based on content, precision, and usage pattern
• Consider platform-specific differences (MySQL vs PostgreSQL vs SQL Server)
• Leverage special data types when working with UUIDs, JSON, or files

⚙️ Real-World Relevance:
Well-defined data types improve storage efficiency and reduce bugs in production systems. They form the backbone of APIs, analytics, and transaction processing.

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❓ FAQ – SQL Data Types

❓ What is the difference between CHAR and VARCHAR?

✅ CHAR is fixed-length; VARCHAR is variable-length. Use CHAR for consistent-length codes like country abbreviations.

❓ Why should I use DECIMAL instead of FLOAT for currency?

✅ DECIMAL provides exact precision, while FLOAT is approximate and may introduce rounding errors—critical in financial data.

❓ Can I use BOOLEAN in all SQL databases?

❌ Not always. Some systems (like MySQL) simulate BOOLEAN using TINYINT(1) or BIT.

❓ What is the max size of VARCHAR?

✅ Depends on the DBMS. For example:

• MySQL: up to 65,535 bytes (depending on row size)
• SQL Server: use VARCHAR(MAX) for large strings

❓ When should I use BLOB or TEXT?

✅ Use them for storing large objects—images, documents, or unstructured text—especially when they’re not often queried.

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