[Introduction to Database System] Summary of SQL Statements

1. The basic concept of SQL

SQL is the abbreviation of Structured Query Language, which means Structured Query Language, which
is a language for querying or changing the data in the database management system

• Data definition language DDL (Data Ddefinition Language)
  SQL data definition language is mainly used to define logical structures, including defining base tables, views and indexes. delete table definition table modify table
• Data Query Language DQL (Data Query Language) The data query language of SQL is mainly used to query various data objects in the database.
• Data Manipulation Language DML (Data Manipulation Language)
  SQL data manipulation language, used to change the data in the database, including inserting, deleting, modifying
• Data control function DCL (Data Control Language)
  SQL data control language, authorization to tables and views, description of integrity rules, and control statements such as transaction start and end.

Second, the basic syntax of SQL DDL

2.1 SQL data types

2.2 Schema definition & deletion

Schema: A layer of definition between database and tables.

2.3 Table definition & deletion & modification

2.4 Index creation & modification & deletion

When the amount of data is relatively large, the query takes a long time. Building an index can effectively reduce the time consumption. An
index can be built on one or more columns

Three, the basic syntax of SQL DQL

Now there are the following student course transcripts

3.1 Query all & query some columns

3.2 Rename the column

select name 姓名 from student；
select name as 姓名 from student；

3.3 Result deduplication

select distinct title from student

3.4 Add conditions to query results

Note that during fuzzy query,


add desc when searching in descending order, if not add the default ascending order

3.5 Use of aggregate functions

select count(title) from edu_course; 
select count(distinct title) from edu_course;//去除重复再数个数
select AVG(price) from edu_course;
select Max(price) from edu_course;
select MIN(price) from edu_course;

3.6 Group query

3.7 Connection

3.7.1 Equivalent join

3.7.2 Self connection

3.7.3 Outer joins

Both are used when connecting sub-tables. If the field names used in the on clause are the same, that is, a.id = b.id, and you don’t want the result to have two id columns , you can use using to simplify Input, using(id)

added:
1.full join full connection: merge the values ​​of left and right connections

2.union all does not deduplicate, union deduplication

3.7.8 Multi-table query

3.7.9 Nested queries

3.7.10 Subqueries with any all

Any:>Any means greater than at least one value, i.e. greater than the minimum value.
All: >All means greater than every value. In other words, it means greater than the maximum

3.7.11 having queries

3.7.12 Set query

And:

cross:
difference:

Fourth, the basic syntax of SQL DML

4.1 Data insertion & deletion & modification

insert:

modify:

delete:

Five, the basic syntax of SQL - data control function DCL

5.1 View

A view (VIEW) is also called a virtual table, that is, a virtual table. It is a logical representation of a set of data. Its essence is that a SELECT result set is assigned a
name, that is, the name of the view.
The view itself does not contain any data, it only contains a query statement mapped to the base table. When the data in the base table changes, the purpose of the view: to facilitate and
simplify data operations.
When our business needs to find multiple tables Data, at this time we may associate multiple tables for query processing.

Change the original table by updating the view: