SQL Basic Keyword Function association connection

Some SQL in the http://www.w3cschool.cn/sql_having.html

1 ORDER by sort

SELECT  from ORDER  by Company;

2 distinct de-weight

SELECT DISTINCT  from Orders;

3 Limit control number of selected bars (MYSQL)

SELECT *  from  5;

ORACLE

SELECT *  from WHERE <= 5;

4 wildcard characters

SELECT *  from  WHERElike'c_r_er'--  _ represents a character 

SELECT *  from WHERE  like ' [aln]% ' -- A% beginning with a or L or N represents one or more characters SELECT *  from WHERE  like ' [! aln]%'--  not starting with a or L or n

5 between

SELECT *  from WHERE  not between ' Adams '  and ' Carter ';

6 Union and UNION ALL

Employees_china:

e_id	E_name
01	Zhang, Hua
02	Wang, Wei
03	Carter, Thomas.
04	Yang, Ming

Employees_usa:

e_id	E_name
01	Adams, John.
02	Bush, George.
03	Carter, Thomas.
04	Gates, Bill.

SELECT  from Employees_china UNION SELECT  from Employees_usa;

Results

E_name
Zhang, Hua
Wang, Wei
Carter, Thomas.
Yang, Ming
Adams, John.
Bush, George.
Gates, Bill.

Two table Overlay Union is more than union all a row weight operation

7 Internal Connection External connection

A table                b
ID      name            ID      
1          a                1       
2          b                3       C
4          C
The inner connection is the same data as the left table and the right table, and the query results have only equal data:
SELECT * FROM A inner join B on A.id=b.id

SELECT * from a b where a.id=b.id

ID Name ID Name
1 a 1 b
Outer joins: Left outer connection, right outer connection, full outer connection
Left outer connection is to left table, to match the right table, the left table has how many data, the result is how many data
SELECT * from A LEFT join B on A.id=b.id
ID Name ID Name
1 a 1 b
2 b NULL NULL
4 c NULL NULL
The right outer connection is connected with the left outer connection, in contrast, to the right table, to match the left table, the right table has how many data, the result is how many data
SELECT * from A right join B on A.id=b.id
ID Name ID Name
1 a 1 b
NULL NULL 3 C
Total outer JOIN Data bar number is not necessarily, quite with is the left outer connection and the right outer joins the synthesis
SELECT * from A full join B on A.id=b.id
ID Name ID Name
1 a 1 b
2 b NULL NULL
NULL NULL 3 C
4 c NULL NULL

 8 UNIQUE constraint constraint also has primary key foreign key default check (id>0, etc.) is not empty

CREATE TABLEPersons (id_pint  not NULL, LastNamevarchar(255) not NULL, FirstNamevarchar(255), Addressvarchar(255), Cityvarchar(255),UNIQUE(id_p))--constrain MYSQL when building tables

CREATE TABLEPersons (id_pint  not NULL UNIQUE, LastNamevarchar(255) not NULL, FirstNamevarchar(255), Addressvarchar(255), Cityvarchar(255)) --ORACLE

9 Index

CREATE INDEX  on DESC);

The role and advantages of database indexing disadvantages

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SQL Basic Keyword Function association connection