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Data Types in SQL: INT, VARCHAR, DATE and More

💡 Concept Name

SQL Data Types define the kind of data a column can hold—such as integers, strings, dates, or booleans. Choosing the right type improves performance and enforces proper data handling.

📘 Quick Intro

Every column in an SQL table must have a data type, determining the kind of values it can store. For example, INT stores whole numbers, VARCHAR stores text, and DATE stores calendar dates. These types influence storage size, operations, and indexing.

🧠 Analogy / Short Story

Think of a filing cabinet where each drawer is labeled for a specific item—files, keys, photos, or documents. You wouldn’t store soup in a drawer for papers! Similarly, SQL data types tell the database what kind of "item" (value) belongs in each column. They help keep order and avoid confusion when storing or retrieving data.

🔧 Technical Explanation

  • INT: Whole numbers (e.g., 1, -100, 42)
  • VARCHAR(n): Variable-length text up to n characters
  • CHAR(n): Fixed-length text of exactly n characters
  • DATE: Stores calendar dates (YYYY-MM-DD)
  • DATETIME: Includes both date and time
  • BOOLEAN: True or false values (usually 0 or 1)
  • FLOAT: Approximate floating-point numbers
  • DECIMAL(p,s): Exact numeric values with precision and scale
  • TEXT, BLOB: Large text or binary objects

🎯 Purpose & Use Case

  • ✅ Store customer names using VARCHAR
  • ✅ Record product prices with DECIMAL
  • ✅ Save birthdates using DATE
  • ✅ Track boolean flags like isActive with BOOLEAN
  • ✅ Use INT for counting orders or IDs

💻 Real Code Example

CREATE TABLE Users (
    UserID INT PRIMARY KEY,
    Username VARCHAR(50) NOT NULL,
    BirthDate DATE,
    Email VARCHAR(100) UNIQUE,
    IsActive BOOLEAN DEFAULT 1,
    AccountBalance DECIMAL(10,2)
);

❓ Interview Q&A

Q1: What is the difference between VARCHAR and CHAR?
A: VARCHAR is variable-length, saving space for shorter strings, while CHAR always uses the full defined length, padding with spaces.

Q2: When should you use DECIMAL over FLOAT?
A: Use DECIMAL for precise financial values, and FLOAT for approximate scientific calculations.

Q3: What does a BOOLEAN store in SQL?
A: It stores TRUE or FALSE values, usually represented as 1 or 0 in many databases.

Q4: How is a DATE different from a DATETIME?
A: DATE only stores calendar dates, while DATETIME stores both date and time.

Q5: Can we use TEXT instead of VARCHAR?
A: TEXT is used for very large strings and doesn’t support indexing or certain functions like VARCHAR does.

Q6: What is the default size for INT?
A: Typically 4 bytes, allowing values between -2,147,483,648 to 2,147,483,647.

Q7: What is the maximum length of a VARCHAR in MySQL?
A: 65,535 bytes (depends on character set and other columns).

Q8: Is BOOLEAN a standard SQL type?
A: Not in all dialects—some emulate BOOLEAN using TINYINT(1).

Q9: Can you store time zones in DATE or DATETIME?
A: No, for that you typically use TIMESTAMP WITH TIME ZONE in supported databases.

Q10: What type should you use to store money?
A: Use DECIMAL or MONEY type if your DBMS supports it.

📝 MCQs

Q1. Which type is best for storing names?

  • INT
  • BOOLEAN
  • VARCHAR
  • DATE

Q2. What does DECIMAL(10,2) mean?

  • Max 10 rows
  • 10 digits total, 2 decimals
  • 10 decimals only
  • Fixed to 2 digits

Q3. Which type stores true/false?

  • VARCHAR
  • BOOLEAN
  • DATE
  • INT

Q4. Which type is fixed-length text?

  • VARCHAR
  • TEXT
  • CHAR
  • BLOB

Q5. What is DATE used for?

  • Phone numbers
  • Calendar dates
  • Passwords
  • Table names

Q6. FLOAT stores what kind of numbers?

  • Exact decimals
  • Strings
  • Approximate decimals
  • Binary only

Q7. VARCHAR(50) means?

  • 50 digits
  • 50 tables
  • Text up to 50 characters
  • 50 bytes exactly

Q8. What type to use for monetary values?

  • VARCHAR
  • BOOLEAN
  • FLOAT
  • DECIMAL

Q9. Which type supports large text blobs?

  • DATE
  • INT
  • TEXT
  • BOOLEAN

Q10. What is stored in DATETIME?

  • Only date
  • Only time
  • Table structure
  • Date and time

💡 Bonus Insight

Choosing the right data type impacts database size, query speed, and data integrity. Always pick the narrowest type that fits your data—like using TINYINT instead of INT for age—or CHAR(2) for fixed-length country codes. It makes databases more efficient and reliable.

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