relational database SQL

sql basic functions

SQLde basic concepts

Main knowledge points

1. The outer schema contains several views and some basic tables
2. The schema contains several basic tables
3. The inner schema contains several storage files
4 Operation objects

Basic table: a table that exists independently, a relationship corresponds to a basic table

View : Only the definition of the view is stored in the database, and the data corresponding to the view is not stored
```
CREATE VIEW view_name AS
SELECT column_name(s)
FROM table_name
WHERE condition
```
Index :
```
CREATE INDEX indexName ON mytable(username(length));
```
5. Define the schema
```
`create schema<模式名> authorization<用户名>`
```
6. Delete mode:

drop schema<模式名> <cascade|restric>
cascade: cascade, which means that all database objects in the schema are deleted when the schema is deleted;
restrict: restriction, which means that the subordinate database objects (such as tables, views) have been defined in the schema, and the deletion statement is rejected. implement

7. Definition, deletion and modification of basic tables
1. Creating a schema establishes a database namespace Each frame, the first thing to define in this frame is the basic database tables contained in the schema.
Create a table:
```
    Create table Student(
    Sno char(9) primary key,
    Sname char(20) unique,
    Ssex char(2)),
    Foreign key(sno) reference SC(Sno), --注意表级约束条件与行级约束条件
```
2. Modify the basic table
```
Alter table <表名>  
[add [column]<新列名><数据类型>  
[drop [column]<列名><cascade|restric>
```
3. Delete the base table
```
    `Drop table <表名>[cascade|restric]`
```
The base table to be dropped cannot be referenced by constraints on other tables

8. Data query
```
select select_list
[into new_table_name]
from table_source
[where search_conditions]
[group by group_by_expression]
[having search_conditions]
[order by order_expression [ASC|DESC]]
```
9. Character matching
%: represents any length a%b
(underscore) represents a single character a_b If it exists, use _ instead
. If the matching character after like does not contain wildcards, you can use =

10.Order by: Sort the search results in ascending or descending order of one or more attribute columns

11. Aggregate function
count(*): count the number of tuples
count(<column name>): count the number of values in a column
sum, avg, max, min When the aggregate function encounters a null value, it skips non-null values value, and only deal with null values
where clause cannot use aggregate function as conditional expression

12.group by: Group the query results by the values of one or more columns, and the values are equal to a group

13.The difference between where and having is the difference in the role of the object

14. Join queries include
```
    等值连接查询:
    非等值连接查询：     嵌套循环连接算法的基本思想
        where S.sno = SC.sno
    自然连接查询：在等值连接中把目标列中重复的属性列去掉则为自然连接
    自身连接查询：查询每一门课的间接先修课
    外连接查询 ：把悬浮元组保存在结果关系中
    复合条件查询：
```
15. Nested query:
the Select statement of the subquery cannot use the order by statement, and the order by can only sort the results
16. Uncorrelated subquery: In
correlated subquery (depending on a property value of the parent query): Exists: Subquery with exists predicate returns no results, only logical true values

17. Collection operations:
```
并操作：union  
交操作：intersect  
差操作：except  
```
18. Query based on derived table: put the subquery in the from clause, you must specify an alias for the derived table

Data Update

1. Insert data
2. Modify data:
```
Update Student 
set sage = 22,
where sno =  '1213223'  
```
3 Delete data
```
delect from 表名 where <条件>
```
4/22/2018 8:40:36 PM

Learning of SQL_sql language

relational database SQL

sql basic functions

SQLde basic concepts

Data Update

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