Knowledge of dark horse database

Table relations

one-on-one , 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 , Add more foreign keys , Associated primary key

Many to many , Realization way ： Create a third intermediate table , The middle table contains at least two foreign keys , Associate the primary keys of two parties respectively

Multi-table query ：

The cartesian product ： take A,B Set all combinations

Multi-table query ： Query data from multiple tables

   ① Link query

        Internal connection ： It is equivalent to a query A,B Intersection data

        External connection ：① The left outer join ： It is equivalent to a query A Table all data and intersection part data

                      ② Right connection ： It is equivalent to a query B Table all data and intersection part data

    ② Subquery

Inner join query syntax

① Implicit inner join

select Field list from surface 1, surface 2...   where Conditions ;

② Display inner connection

select Field list from surface 1 [inner]  join surface 2 on Conditions

example ：select * from emp inner join dept on emp.dep_id = dept.did;

1. The left outer join ： It is equivalent to a query A All the data + The intersection part

select Field list from surface 1 left [outer] join surface 2 on Conditions ;

example ：select * from emp left join dept on emp.dep_id = dept.did;

2. Right connection ： It is equivalent to a query B All the data + The intersection part

select Field list from surface 1 right [outer] join surface 2 on Conditions ;

Subquery （ nested queries ）

1. Sub query concept ： Nested query in query , A nested query is called a subquery

2. Sub queries differ according to the query results , The effect is different , It is divided into ：

① Single row single row ： As a condition value , Use =,！=,>,< Wait for conditional judgment

select Field list from surface where Field name =（ Subquery ）

② Multi row single row ： As a condition value , Use in And other keywords for conditional judgment

select Field list from surface where Field name in （ Subquery ）

③ Multiple rows and columns ： As a virtual table

select Field list from （ Subquery ） where Conditions

Business profile ：

① Database transactions （transaction） It's a mechanism , A sequence of operations , Contains a set of database operation commands

② The transaction submits or revokes the operation request to the system as a whole , That is, this set of database commands either succeed at the same time , Or fail at the same time .

③ A transaction is an indivisible unit of work logic

--- Open transaction

begin or start transaction;

--- Commit transaction （ The data is temporary , Submitted is permanent , You can access ）

commit;

--- Roll back the transaction （ Rollback to before starting the transaction ）

rollback;

The interview asked ：

Four characteristics of business （ACID）

① Atomicity （Atomicity）: Transactions are the smallest and indivisible unit of operation , Or at the same time , Or fail at the same time

② Uniformity （consistency）: When the transaction completes , All data must be kept consistent

③ Isolation, （isolation）： Between multiple transactions , Visibility of actions

④ persistence （durability）： Once a transaction is committed or rolled back , Its changes to the data in the database are permanent

MySQL Transactions are automatically committed by default （oracal To submit manually ）

--- View the default commit method for transactions

select @@autocommit;

The result is 1 Automatic submission 0 Manual submission

--- Modify the transaction submission method

set @@autocommit = 0;