SQL — Syntax

SQL is case-insensitive

SQL keywords can be written in any case:

SELECT * FROM students;
select * from Students;
Select * FROM STUDENTS;

All three are identical. Convention is to write keywords in UPPERCASE and table/column names in lowercase:

SELECT name, age FROM students WHERE grade = 'A';

Statements end with semicolons

Each SQL statement ends with a semicolon:

SELECT * FROM students;
INSERT INTO students (name) VALUES ('Ada');
UPDATE students SET age = 25 WHERE name = 'Ada';

Some tools don't require the semicolon for single statements, but always include it — it's required when running multiple statements.

The order of clauses

SQL clauses must appear in a specific order:

SELECT columns
FROM table
WHERE condition
GROUP BY columns
HAVING condition
ORDER BY column
LIMIT number;

Not all clauses are required. The minimum query is:

SELECT * FROM students;

SELECT — choosing columns

-- All columns
SELECT * FROM students;

-- Specific columns
SELECT name, age FROM students;

-- No duplicates
SELECT DISTINCT grade FROM students;

-- Column aliases
SELECT name AS student_name, age AS student_age FROM students;

-- Expressions
SELECT name, age + 1 AS next_year_age FROM students;

FROM — specifying the table

SELECT * FROM students;
SELECT * FROM students AS s;

The FROM clause tells SQL which table to read from. Use aliases for shorter queries when joining tables.

WHERE — filtering rows

SELECT * FROM students WHERE age > 20;
SELECT * FROM students WHERE grade = 'A';
SELECT * FROM students WHERE age >= 18 AND age <= 25;
SELECT * FROM students WHERE name LIKE 'A%';
SELECT * FROM students WHERE grade IN ('A', 'B');
SELECT * FROM students WHERE age BETWEEN 18 AND 25;
SELECT * FROM students WHERE name IS NOT NULL;

ORDER BY — sorting

-- Ascending (default)
SELECT * FROM students ORDER BY name ASC;

-- Descending
SELECT * FROM students ORDER BY age DESC;

-- Multiple columns
SELECT * FROM students ORDER BY grade ASC, age DESC;

LIMIT — restricting results

-- First 5 rows
SELECT * FROM students LIMIT 5;

-- Skip 10, return 5
SELECT * FROM students LIMIT 5 OFFSET 10;

INSERT — adding data

-- Single row
INSERT INTO students (name, age, grade)
VALUES ('Ada', 20, 'A');

-- Multiple rows
INSERT INTO students (name, age, grade)
VALUES ('Bob', 22, 'B'), ('Charlie', 19, 'A');

UPDATE — modifying data

UPDATE students
SET grade = 'A-', age = 21
WHERE name = 'Bob';

DELETE — removing data

DELETE FROM students WHERE name = 'Charlie';
DELETE FROM students;  -- removes ALL rows (careful!)

Data types

Type	Description	Example
INT	Whole number	42
VARCHAR(n)	Text up to n characters	'Hello'
TEXT	Long text	paragraphs
DECIMAL(p,s)	Precise decimal	9.99
FLOAT	Approximate decimal	3.14
DATE	Date only	'2024-01-15'
DATETIME	Date and time	'2024-01-15 14:30:00'
BOOLEAN	True/false	TRUE
BLOB	Binary data	images

NULL values

NULL means "missing" or "unknown" — not zero, not empty string:

-- Check for NULL
SELECT * FROM students WHERE age IS NULL;
SELECT * FROM students WHERE age IS NOT NULL;

-- NULL in comparisons
SELECT * FROM students WHERE age = NULL;    -- WRONG! Returns nothing
SELECT * FROM students WHERE age IS NULL;   -- CORRECT

Comments

-- Single line comment

/*
   Multi-line comment
   spans several lines
*/

SELECT * FROM students; -- inline comment

SQL's structure

Think of a query as a pipeline:

FROM    →   which table to read
WHERE   →   which rows to keep
SELECT  →   which columns to show
ORDER BY →  how to sort the results

The database executes these steps in a specific order, not the order you write them. Understanding this helps you write efficient queries.

Mini Practice

1. Write a query that selects name and age from a table, sorted by age descending
2. Insert three rows with different data types
3. Update all rows where a condition is true
4. Delete rows matching a specific condition
5. Write a query with WHERE, ORDER BY, and LIMIT together

Next: the SELECT statement in depth →