Simple cases of inner connection and left connection

Inner connection is also called equivalent connection , The result set returned is all the matching data in the two tables , And discard the mismatched data . in other words , In this kind of query ,DBMS Return only related rows from the source table , That is, the result table of the query contains two source table rows , Must satisfy ON Search criteria in clause . As a contrast , If the row in the source table has no corresponding row in another table , Then the line is filtered out , Not included in the results table . Inner joins are done using comparison operators . There are two types of internal connections .

1. Equivalent connection 
 Use the equal sign （=） Compare column values of connected columns , List all the columns in the linked table in the query result , Include the repeating Columns .

Case study ： Use INNER JOIN Internal connection query
from STUDENT Table and TEACHER Query student name in the table , Department , The course number of all courses taken and the name of the teacher .

SELECT SNAME,S.DNAME,S.CNO,TNAME
FROM STUDENT AS S
INNER JOIN TEACHER AS T
ON S.CNO = T.CNO
ORDER BY SNAME

Next, write a simple case of left connection

The so-called left connection , namely LEFT JION Clause , It allows us to query data from two or more database tables .
LEFT JION Clause yes SELEC Optional parts of the statement , It appears in FROM After Clause .

Let's assume that we need to start from two tables t1 and t2 Query data . The following statement illustrates the connection of two tables LEFT JOIN Syntax of clause ：

SELECT 
	t1.c1,t1.c2,t2.c1,t2.c2
FROM
	t1	
		LEFT JOIN
	t2 ON t1.c1 = t2.c1;

When using LEFT JOIN The clause will t1 Table join t2 Table time , If it comes from the left table t1