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Core Code Modern Dev Handbook

SQL Data Types

SQL data types define what kind of values a column can store. Choosing the right types improves data quality, storage, and performance.
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SQL Data Types

SQL data types define what kind of values a column can store. Choosing the right data types improves data quality, reduces storage, and can improve performance.

Common Categories

  • Numeric types (INT, DECIMAL, FLOAT)
  • String types (CHAR, VARCHAR, TEXT)
  • Date and time types (DATE, DATETIME, TIMESTAMP)
  • Boolean-like types (often implemented as TINYINT)

Integer Types

Use integer types for whole numbers (ids, counters, quantities).

id INT AUTO_INCREMENT PRIMARY KEY

Exact Numbers (DECIMAL)

Use DECIMAL for money and values that require exact precision.

price DECIMAL(10,2) NOT NULL

Approximate Numbers (FLOAT/DOUBLE)

FLOAT and DOUBLE are approximate and can introduce rounding errors. Use them for scientific measurements, not money.

String Types

  • CHAR(n) – fixed-length text (good for fixed-size codes)
  • VARCHAR(n) – variable-length text (most common choice)
  • TEXT – larger text content

Example: VARCHAR vs TEXT

name VARCHAR(100) NOT NULL,
description TEXT

Date and Time Types

  • DATE – stores a date (YYYY-MM-DD)
  • DATETIME – stores date and time
  • TIMESTAMP – stores date and time (behavior depends on database/timezone settings)

Example: Timestamps

created_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP

Boolean Values

Some databases have a BOOLEAN type, but in MySQL/MariaDB it is commonly stored as TINYINT(1).

is_published TINYINT(1) NOT NULL DEFAULT 0

Choosing the Right Size

Do not oversize columns. For example, using VARCHAR(5000) for short values wastes resources and can hurt indexing.

Character Set (utf8mb4)

Use utf8mb4 for full Unicode support. Make sure your database and tables use compatible collation.

Common Mistakes

  • Storing numbers as text
  • Using FLOAT for money
  • Using very large VARCHAR sizes without need
  • Ignoring character set and collation

Best Practice

Choose the smallest appropriate type that fits your data. Use constraints (NOT NULL, UNIQUE) to enforce quality and use indexes for columns frequently searched or joined.

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