I am accustomed to using SQL Sever to directly design the database every time, but do not pay enough attention to SQL statements. I took the opportunity of this reconstruction of the computer room cooperation version to use SQL statements to design the database and at the same time Let’s sort out the basic usage of SQL statements.
There are 9 types of SQL statements, which can be roughly divided into 4 types according to their functions:
1. Query statement: select ----The most commonly used statement in the database
2. Data definition: create, drop, alter----for Operations on databases, data tables, and fields in tables
3. Data manipulation: insert, update, delete----the legendary "add, delete, modify, query", for specific Data operations
4. Data control: grant, revoke----definition and deletion of permissions
1. Create a database
##
create database RechargeSystem on primary --主文件组 ( /*数据文件的具体描述*/ name='RechargeSystem_data', --主数据文件的逻辑名称 filename='G:\RechargeSystem\RechargeSystem_data.mdf', --主数据文件的物理名称 size=5mb, --初始大小 maxsize=100mb, --增长最大值 filegrowth=15% --主数据文件增长率 ) log on ( /*日志文件的具体描述*/ name='RechargeSystem_log', filename='G:\RechargeSystem\RechargeSystem_log.ldf', size=2mb, filegrowth=1mb )If you have learned the operating system, you will not be very familiar with these definition statements. Strangely enough, a database group file is also a kind of file and needs to have a logical name (i.e. file name), physical name (actual storage address on the disk), file size, etc.; the log file records all logical operations in the database, with details such as advantages and disadvantages. For the content, see Baidu Encyclopedia: Log file
2. Basic table operations
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create table <基本表名> ---建立表格
（<列名 类型 not null>，
primary key（列名，列名），--主键约束
foreign key （<列名>） references tableName(<列名>)，--外键约束）；
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alter table <表名> add <列名><数据类型> --添加列
You can set the primary and foreign keys of the established table:
## alter table <表名> add primary key(<列名>,<列名>)
alter table<表名> add foreign key(<列名>) references tableName（<列名>）
## drop tableccc43248daffbac9770dee47fdaff697[cascade|restrict] --Delete table, with constraints Restrict
must be added to the condition
3. Query statement:
Regular The query statement is: select * from datatable
It should be noted that nested and connected queries are queried from multiple tables. This combination relationship is commonly used in M:N relationships. Find the corresponding relationship.
select TB1.column1,TB1.column2
from TB1
where TB1.column1 in（select column1
from TB2
where TB2.column2="特定值"）；
This relationship is not recommended for users with confusing logic, as it is particularly likely to cause search failure.
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In addition, the SQL query statement also provides basic aggregate functions for easy query use:
count(*) --- --------------------Calculate the number of tuples, that is, how many columns there are in total
count(f36792d11432c88ac43f2af3b024a3a3)----------------Calculation of the number of values in a column
sum(4856a503a57d954e03d677c32e973360)-- ------------------The sum of a column of values (numeric type)
avg (be11c19f69948f0d95b6cf7d7a9192c3)-- -------------Average (must be numeric type)
max(4856a503a57d954e03d677c32e973360)-------- ---------Maximum value
## min(4856a503a57d954e03d677c32e973360)------------------ ----Minimum value
#--------------------------------------------- -------------------------------------------------- --------------------------------------------------
Complete semantic structure:
’’’''‐‐' '' #''''''''‐'‐'‐'‐‐‐‐---- where 081f1dd6115c2b762fda5face608cc63 ## group by e147dec42e83e3b84883e9b9d2f9cc02 ----------------------- ---------Grouping based on a certain field ## Order by 7f82678259a28ffd9ad874ce1622ed47 ---------- --------Asc ascending order, desc descending order ----------------------- -------------------------------------------------- -------------------------------------------------- ------------------- ## Special query requirements: 1. Comparison Operations: 731221495c70ac245ce7f5323824b5f2,>=,!= ## 2. Matching operation: %---matches characters consisting of zero or more characters , "_" single character matches 3. Null value is Null 4. Aggregated resource comparison: in, some, any, all 5. Whether the set is non-empty: exits --- If it is not empty, it is true, otherwise it is false 6. Whether there are duplicates in the set Tuple: unique--------repeat to true, otherwise false -------------------------- -------------------------------------------------- -------------------------------------------------- ----------------------- 4. Data operation statements Insert: insert into ccc43248daffbac9770dee47fdaff697 (e147dec42e83e3b84883e9b9d2f9cc02, e147dec42e83e3b84883e9b9d2f9cc02...) values('value', 'value') Delete: delete from ccc43248daffbac9770dee47fdaff697 where condition Modify: update ccc43248daffbac9770dee47fdaff697 set [Summary] Knowledge is always like this , you will gain different results every time you study, especially after using it, you will try to compare the differences between different methods. I look forward to the next comparative study of database views, stored procedures and security settings.
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