SQL & DatabasesNot Started
SQL SELECT
Query data with SELECT, WHERE, ORDER BY, LIMIT
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Knowledge Debt detected
You can study this freely — but your score may plateau if these foundations have gaps. The Mastery badge requires them to be solid.
Partial Coverage
This skill is being actively expanded. You'll get real value from what's here, but it doesn't cover everything yet.
Available now:
- SELECT, WHERE, ORDER BY, LIMIT
- JOINs (INNER, LEFT, RIGHT)
- Aggregates & GROUP BY
- Indexes & Performance
Coming soon:
- Transactions & ACID
- Database Design & Normalization
- Window Functions
- Query Optimization with EXPLAIN
Explanation
SELECT retrieves data from database tables.
sql
SELECT name, age
FROM users
WHERE age > 18
ORDER BY name ASC
LIMIT 10;Key clauses (in order):
- 1.
SELECT— which columns to return (* = all) - 2.
FROM— which table - 3.
WHERE— filter rows (optional) - 4.
ORDER BY— sort (ASC/DESC) - 5.
LIMIT— cap the number of results
DISTINCT removes duplicates: SELECT DISTINCT city FROM users
Examples
Filter and sort
Find top 5 highest-paid engineers
SELECT id, name, salary
FROM employees
WHERE department = 'Engineering'
AND salary > 80000
ORDER BY salary DESC
LIMIT 5;Next in SQL & Databases
SQL JOINs