Database learning summary 5

One 、 Multi-table query

1. grammar ：

select

        List of names

from

        List of table names

where...

The cartesian product ： Yes A,B Two sets , All the components of these two sets ;

To complete a multi table query , Need to eliminate useless data

2. Classification of multi table query

（1） Internal connection query

First, make sure ：

        Query data from those tables

        What are the conditions

        Which fields to query

a. Implicit inner join ： Use where Condition clear useless data

select Field list from List of table names where Table name 1.` Name ` = Table name 2.` Name `;

The name fields of two tables may be repeated , You can use table name punctuation before column name to distinguish ;

But it's going to be a lot of trouble , You can alias a table

select  

        t1.name,

        t1.age,

        t2.name

from

        Table name 1 t1,

         Table name 2 t2

where

        t1.` Name ` = t2.` Name `;

Each field has only one line, so it is convenient to add comments after it

b. Explicit inner connection ：

select Field list from Table name 1 【inner】 join Table name 2 on Conditions ;

【inner It can be omitted 】 

（2） External connection query

a. The left outer join : It queries all the data in the left table and its intersection part

select Field list from Table name 1 left 【outer】 join Table name 2 on Conditions ;

【outer It can be omitted 】 

b. Right connection ： It queries all the data in the left table and its intersection part  

select Field list from Table name 1 right 【outer】 join Table name 2 on Conditions ;

【outer It can be omitted 】 

（3） Subquery ： Nested query in query

a. The result of subquery is single row and single column ：

         Subqueries can be used as conditions , Use operators to judge （>、<、>=、<= Isomorphic operator ）

b. The result of subquery is multi row and single column ：

         Subqueries can be used as conditions , Use operators to judge （   in And so on ）

c. The result of subquery is multi row and multi column ：

        A subquery can be used as a Virtual table

Two 、 Business

1. Basic introduction to the business

（1） Concept ： If a business operation contains multiple steps , Managed by affairs , So these operations are either successful at the same time , Or fail at the same time .

（2） operation

a. Open transaction ：start transaction;

b. Roll back ：rollback;

c. Submit ：commit;

（3) Two ways to commit a transaction

        a. Automatic submission ：mysql It's automatic submission ,  One DML（ Additions and deletions ） Statement will be automatically submitted once

        b. Manual submission ： You need to start the transaction before committing

（4） Modify the default submission method

        a. View the default submission method ：SELECT @@autocommit;  -- 1 On behalf of auto submit -- 0 On behalf of hand submit

        b. Modify the default submission method ：SET @@autocommit = 0;

2. The four characteristics of affairs

（1） Atomicity ： Is an indivisible minimum operating unit , Or at the same time , Or fail at the same time .

（2） persistence ： When a transaction is committed or rolled back , The database will persist the data .

（3） Isolation, ： Between many things , Are independent of each other .

（4） Uniformity ： Before and after transaction operation , The total amount of data remains the same .

3. The isolation level of the transaction （ understand ）

（1） Concept ： Isolated... Between multiple transactions , Mutually independent . But if multiple transactions operate on the same batch of data at the same time , It will cause some problems , Setting different isolation levels can solve these problems