SQL: Structured Query Language

1. Basic concepts

• A database is a collection of tables.
• A table is like an Excel sheet: it has columns (fields) and rows (records).
• SQL is used to create, query, insert, update and delete data.

Example of a Clients table:

2. Basic queries (SELECT)

The most important command is SELECT, which is used to read data:

-- Select all columns
SELECT * FROM Clients;

-- Select only some columns
SELECT Name, City FROM Clients;

-- Filter data (only those from Girona)
SELECT * FROM Clients
WHERE City = 'Girona';

-- Order data by age
SELECT * FROM Clients
ORDER BY Age DESC;   -- DESC = descending

3. Insert data (INSERT)

INSERT INTO Clients (ID_Client, Name, City, Age)
VALUES (4, 'Jordi Pons', 'Lleida', 30);

4. Update data (UPDATE)

UPDATE Clients
SET City = 'Madrid'
WHERE ID_Client = 2;

5. Delete data (DELETE)

DELETE FROM Clients
WHERE ID_Client = 3;

6. Create a table (CREATE TABLE)

CREATE TABLE Clients (
  ID_Client INT PRIMARY KEY,
  Name VARCHAR(50),
  City VARCHAR(50),
  Age INT
);