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.
【Classification】
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
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
[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"）；
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This relationship is not recommended for users with confusing logic, as it is particularly likely to cause search failure.
set ##5. Embedded SQL usage technology, commonly used execution views, stored procedures, etc., add exec to execute
[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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