SQL database → constraint → design → multi table query → transaction

constraint

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1. The concept of constraints
Constraints are rules that act on columns in a table , Used to limit the data added to the table
The existence of constraints ensures the correctness of elements in the database
2. Classification of constraints
Foreign key constraints （ The physical connection ）

Database design

What is database design ？
◆ What watches are there
◆ What fields are there in the table
◆ What is the relationship between tables
What kinds of table relationships are there ？

notes ：ER chart （Entity Relationship)
eg:( Forum system ER chart ）
Table relations
* one-on-one
* Such as ： User and user details
* One to one relationships are mostly used for table splitting , Put the fields often used in an entity in a table , Infrequently used fields are placed in another table , Used to improve query performance
● Realization way ： Add foreign keys on either side , Associate another party's primary key , And set the foreign key to be unique （UNIQUE）

* One to many （ For one more ）
* Such as ： Departments and employees
* A Department corresponds to multiple employees , One employee corresponds to one department *
● Realization way ： Build foreign keys on more than one side , A primary key that points to a party of one

* Many to many ：
* Such as ： Goods and orders
* One item corresponds to multiple orders , An order contains multiple items
● Realization way ： Create a third intermediate table , At least two foreign keys in the middle , Associate the primary keys of two parties respectively

Database design case

analysis

Multi-table query

1. Inner join query syntax

2. External connection query syntax

3. Subquery ：
（ Nested queries are called subqueries in queries ）

Business

● Database transactions （Transaction） It's a mechanism 、 A sequence of operations , Contains a set of database operation commands
● Transactions submit or revoke requests to the system with all commands as a whole , That is, this set of database commands either succeed at the same time , Or fail at the same time
● Transaction is an indivisible logical unit
—— Open transaction
START TRANSACTION; perhaps BEGIN;
—— Commit transaction
COMMIT;
—— Roll back the transaction
ROLLBACK;

Four characteristics of business （ Frequently asked questions ）