SQL based series --- left join

1.left join will be left to return all the rows in the table, even if the right table does not match the line.
2.LEFT JOIN keyword syntax
SELECT column_name(s)
FROM table_name1
LEFT JOIN table_name2 
ON table_name1.column_name=table_name2.column_name

Note: In some databases, LEFT JOIN is called LEFT OUTER JOIN.
The original table (used in the case of):
"Persons"  Table  :
Id_P    LastName        FirstName       Address City
1       Adams   John    Oxford Street   London
2       Bush    George  Fifth Avenue    New York
3       Carter  Thomas  Changan Street  Beijing

"Orders" table:
Id_O  OrderNo Id_P
1       77895   3
2       44678   3
3       22456   1
4       24562   1
5       34764   65

Left connection (LEFT JOIN) instances now, we hope all the people listed and their order - if any.
You can use the following SELECT statement:
SELECT Persons.LastName, Persons.FirstName, Orders.OrderNo
FROM Persons
ON Persons.Id_P=Orders.Id_P
ORDER BY Persons.LastName

Result set:
LastName   FirstName       OrderNo
Adams   John    22456
Adams   John    24562
Carter  Thomas  77895
Carter  Thomas  44678
Bush    George   

LEFT JOIN keywords from the left table (Persons), where all the rows returned, even in the right table (Orders) does not match the line.
标签: column name, databases, select statement, instances, table id, address city, oxford street london, bush george, fifth avenue new york
分类: Database
时间: 2010-09-26


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