Data Types

A data type is the kind of value a column can hold. INT is a whole number (roll, marks). VARCHAR(50) is short text (name). DATE is a calendar day. DECIMAL(10,2) is money — not FLOAT, because 0.1 + 0.2 in binary floats is messy.

Pick the smallest type that fits. Marks 0–100 → TINYINT or INT, not VARCHAR. Phone as VARCHAR, not INT — leading zero and +91 will break. ENUM is a short fixed list (‘pass’,‘fail’) — don’t use it for a list that grows every week.

NULL means ‘unknown’, not zero and not empty string. A marks column can be NULL if the exam is not written yet. Don’t write WHERE marks = NULL. Write WHERE marks IS NULL.

Board: CREATE TABLE students (roll INT PRIMARY KEY, name VARCHAR(50) NOT NULL, marks INT, dob DATE). That one line teaches types better than a chart of 40 names.

Trap: storing money in FLOAT. Trap: VARCHAR(1) for names. Trap: using TEXT for every column ‘just in case’ — indexes and memory suffer.

On the example next to this theory: Data Types: create two demo rows, then SELECT qty >= 2 ordered. Say which labels come back.

Exam tip

INT / VARCHAR / DATE / DECIMAL. Show one CREATE TABLE. NULL ≠ 0.

Data Types — sample query

-- Data Types
CREATE TABLE demo (
  id INTEGER PRIMARY KEY,
  label TEXT NOT NULL,
  qty INTEGER DEFAULT 0
);
INSERT INTO demo (id, label, qty) VALUES
  (1, 'alpha', 2),
  (2, 'beta', 5);
SELECT label, qty FROM demo WHERE qty >= 2 ORDER BY qty DESC;

Data Types: create two demo rows, then SELECT qty >= 2 ordered. Say which labels come back.

Short notes

DefColumn type = what value is allowed (INT, VARCHAR, DATE, DECIMAL).
RuleMoney → DECIMAL. Names → VARCHAR. Unknown → NULL.
RememberWHERE col IS NULL, never = NULL.
TrapFLOAT for money; INT for phone numbers.

Data Types

NULL means ‘unknown’, not zero and not empty string. A marks column can be NULL if the exam is not written yet. Don’t write WHERE marks = NULL. Write WHERE marks IS NULL.

Board: CREATE TABLE students (roll INT PRIMARY KEY, name VARCHAR(50) NOT NULL, marks INT, dob DATE). That one line teaches types better than a chart of 40 names.

Trap: storing money in FLOAT. Trap: VARCHAR(1) for names. Trap: using TEXT for every column ‘just in case’ — indexes and memory suffer.

On the example next to this theory: Data Types: create two demo rows, then SELECT qty >= 2 ordered. Say which labels come back.

Exam tip

INT / VARCHAR / DATE / DECIMAL. Show one CREATE TABLE. NULL ≠ 0.

Data Types — sample query

-- Data Types
CREATE TABLE demo (
  id INTEGER PRIMARY KEY,
  label TEXT NOT NULL,
  qty INTEGER DEFAULT 0
);
INSERT INTO demo (id, label, qty) VALUES
  (1, 'alpha', 2),
  (2, 'beta', 5);
SELECT label, qty FROM demo WHERE qty >= 2 ORDER BY qty DESC;

Data Types: create two demo rows, then SELECT qty >= 2 ordered. Say which labels come back.

Short notes

DefColumn type = what value is allowed (INT, VARCHAR, DATE, DECIMAL).
RuleMoney → DECIMAL. Names → VARCHAR. Unknown → NULL.
RememberWHERE col IS NULL, never = NULL.
TrapFLOAT for money; INT for phone numbers.

Data Types

Data Types — sample query

Short notes

Questions

Data Types

Data Types — sample query

Short notes

Questions