Soft test --- database (4) SQL statement

SQL Language

1.1 Definition

SQL Structured query language （ Structured Query Language ） Abbreviation , Its functions include data query 、 Data manipulation 、 Data definition and data control are four parts ;

• SQL The language is simple 、 Convenient and practical 、 The function is all ready , It has become the most widely used relational database language ;
• SQL It's self-contained language （ Online interaction ）, Embedded language again （ Host language ）;

1.2 Two basic concepts

Basic table ：

Is an independent table , Tables that are not exported from other tables , A relationship corresponds to a basic table , One or more basic tables correspond to one storage file ;

View ：

A view is a virtual table , It is a table exported from one or several basic tables , The database only stores the definition of the view, not the data corresponding to the view , This data is still stored in the base table of the exported view , When the data in the basic table changes , The data queried from the view also changes ;

1.3 Data definition

SQL Language uses data definition language （ DDL ） Realize its data definition function , For database users 、 Basic table 、 View 、 Index to define and undo ;

1.3.1 Define the base table

CREATE TABLE < Table name >( < Name > < data type > < Column level integrity constraints > )

< Table name > Legal identifier , most 128 Characters , Such as S , SC , C , Duplicate names are not allowed ;

1.3.2 Modify the basic table

ALTER TABLE < Table name >

[ ADD < New column names > < data type > [ Integrity constraints ] ]

[ ALTER COLUMN < Name > < data type > ]

[ DROP < COLUMN Name > | < Integrity constraint name > ]

• ADD Clause ： Add new columns and new integrity constraints ;
• ALTER COLUMN Clause ： Used to modify Columns ;
• DROP Clause ： Delete the specified column or integrity constraint ;

1.3.3 Delete table

DROP TABLE < Table name >

After deleting the basic table , The data and the index built on the table are deleted , The view established on this table will not be deleted , The system will continue to retain its definition , But it can no longer be used ;

1.3.4 Define views

CREATE VIEW < View name > [ ( < Name > [ < Name > ] …… ) ]

AS

< Subquery >

[ with check option ]

1.3.5 Delete view

DROP VIEW < View name >

1.4 Query statement

1.4.1 SELECT General format of statement

SELECT [ ALL | DISTINCT ] < Target list expression > [ , < Target list expression > ]

FROM < Table or view name > [ , < Table or view name > ] …… | ( SELECT sentence )

​ [ AS ]< Alias >

[ WHERE < Conditional expression > ]

[ GROUP BY < Name 1 > [ HAVING < Conditional expression > ] ]

[ ORDER BY < Name 2 > [ ASC | DESC ]];

1.4.2 SELECT Implementation process of

• according to WHERE Search condition of clause , from FROM Select the tuples that meet the conditions in the basic table or view specified by clause , Again according to SELECT Clause to get the result table ;
• If there is GROUP Clause . Then the query results will be according to < Name 1 > Group the same values ;
• If DROUP Clause followed by HAVING The phrase , Then only the output meets HAVING Conditional tuple ;
• If there is ORDER Clause , The query results should also be in accordance with < Name 2 > Arrange values of ;

1.4.3 Link query

There are two ways to connect tables ：

• Rows between tables that meet certain conditions are connected , here FROM Clause indicates the name of the table to connect , WHERE Clause indicates the column name of the connection and its connection condition ;
• Using keywords JOIN Connect ;