SQL is a relational database NoSQL is non-relational database

SQL (Structral Query Language)

Basic Concept

Primary Key： The primary key cannot be the same and try not to change
Foriegn Key： The Foriegn key can connect many tables

The SQL Language

Basic Search

Search for every row in the table:

SELECT * FROM <The name of the table>

SELECT 100+200

Calculate 100+200=300

Conditional Search

SELECT * From  <The name of the table>  WHERE  <Condition 1>  AND/OR  <Condition 2>

Projection Search

Search for some columns in the table and can substitute their name

SELECT id, score points, name FROM <The name of the table>  WHERE <Condition>

Sorting

SELECT id, score, name
FROM <The name of the table>
WHERE <Condition>
ORDER BY <the key to be sorted>

Devide the page

SELECT id, score, name
FROM <The name of the table>
WHERE <Condition>
ORDER BY <the key to be sorted> DESC   // in descending order
LIMIT 3 OFFSET 0    //Devide into pages with 3 record in each page, offset = 0 + n * 3

Count the Total Number of the record

SELECT COUNT(*) FROM <The name of the table>

Query From more tables

SELECT * FROM  <The name of table 1>  <The name of table 2>

JOIN

SELECT s.id, s.name, s.class_id, c.name class_name, s.gender, s.score
FROM students s
INNER JOIN classes c
ON s.class_id = c.id;