What does the left table in VFP outer join mean in the right table

For example, if there are two tables, A and B, the result of the left connection of the two tables is that all the contents in table A are displayed, only the records related to table A are displayed in table B, and the fields with records in table A and no records in table B are displayed with null values. The right join is that A only shows the fields related to table B, and table B displays all of them, and the left join is the opposite, and there are records in table B, and fields without records in table A show null values; An inner join shows only the records that are connected in the two tables, a full join shows both table records, and a null value is displayed where there is no common field.

The left table and the right table are quite divided. That is, the table to the left of the keyword jion is the left table, and the right table is the right table.

For example, from a inner join b ona is the left and table b is the right.

The difference between left outreach and right outreach is actually complementary, and it is also based on the left outlink of A and the table B, which in turn is equivalent to the left of B right outreach. Such as from a left inner jion bfrom b right inner jion a left outreach and right outreach. That is, let all the data of the left table (right table) be taken and the data associated with the right table (left table) be found at the same time.

Illustrate. Student Sheet, Examination Score Sheet.

From student left jion result to find all students, and their corresponding grades. However, there may be students who did not take the test, so the corresponding result is null, which means that they are absent from the test.

Don't understand this? That is, the table is an independent self, for example, there is no one in the world who can contain you, and his table can only be this one table. You can join several tables with a common field name. Complete. Go to bed.

Joining tables is mainly used for various queries or to generate various views, while maintaining data integrity between two tables and avoiding data redundancy.

It is used to find filter information after establishing contact. For example, if you want to find all or part of the information in these two tables, use the sqr statement to find them. Finally, find out the results you want through the connections between them.

A form is a form, also known as a window. Forms allow users to view and data in a familiar interactive interface.

Forms offer much more than just an interface. It also provides a rich set of objects that respond to user or system events, making it easy and intuitive for users to manage information.

Intertable joins:

inner join

left join

right join

full join

Relationship between tables: one-to-one and one-to-many.

Many-to-many (not represented in VFP or other databases).

Inner joins, left joins, right joins, full joins.

year() returns the year from the specified date expression.

Database connection command: join with 《alias》 to 《filename》[fiel《field name table」】 for 《connection condition》

Use the primary database records to find the same records in the slave database one by one by the connection expression value (the connection condition is mandatory), and generate the records in the new database. The fields of the new library are generated (horizontally joined) by both libraries

Example 1 (if both databases have student number fields).

Sele 2 opens the Grade Library in Workspace 2.

use cjk

Sele 1 opens the student library in workspace 1.

use xsk

Join with to XSCJ Field Name, Student Number, Class, B-> Chinese, B-> Mathematics, B-> English, Total Score for Student Number=B-> Student Number.

use xscj

listRecord Number Name Student Enrollment Number Class Chinese Mathematics English Total Score The resulting new library contains fields from both databases.

1 Shen Qing 9504001 Gao 1 87 78 88

Omitted).

Merge the two related tables into a temporary table, and then display select table 1 with **Field 1, Table 1Field 2, Table 1Field 3, Table 2Field 1, Table 2Field 2

from Table 1 left join Table 2 on Table 1Field 1, Table 2Field 2

into cursor temporary tables.