issues with left and right connections to the database

select ,from emp e left jion dept don ;

Left Company. The left link is, the left side prevails, and the right side is useless to make up for it.

For example, if there is an employee who does not have a leader, the left link will indicate that the leader's name is empty, indicating all employees, regardless of whether they have a leader or not.

The right link is not The result is that there is no such employee, only the information of the employee with the leader is displayed.

Right Company in reverse.

select ,from emp e right jion dept don ;Right Link.

+) Method connection is a unique connection method in oracle, you shouldn't be executed in oracle, right?

It is recommended that you still use the standard jion method:

select lastname,firstname,ordernofrom persons12

left join orders12

on =

select lastname,firstname,orderno from persons12, orders12

where = just like that.

I don't know what it means.

Inner join: Take the intersection of two tables (with fields that can be joined), that is, the fields are the same. You can use the inner join to get the records of the common part of the two tables, select * from a, b where

The result is the same as the run of select * from a join b on.

External Connection: Left and right connection.

There are two types of outer joins, one is left join and right join

1) Left join: Figure 3 Common part record set C Table A record set A1. The statement is as follows:

The select * from a left join b on a field is all there, and the query is on the left side of the result set.

2) Right join: Figure 3 Common part record set C table B record set B1. The statement is as follows: select * from a right join b on

The format of Data Source is very complex, refer to MSDN, format, different SQL Server versions have different localization.

It is still recommended to use Visual Studio to generate the connection string, click on the data menu - add a new data source, select a type, etc., and finally use it.

string constr=database name].connectionstring;

sqlconnection con=new sqlconnection(constr);

That's it.

That's it.

string connectionstring ="server=(local);database=database name; uid=sa;pwd="

uid is the user name.

PWD is the password.

Table joins. Inner Connections (Natural Connections).

Outer Join (1) Left Outer Join (the table on the left is unlimited).

2) Right outer join (the table on the right is unlimited).

3) Full external connection (no restriction on the left and right tables).

Self-join (join within the same table).

inner join means inner join;

left join indicates the left outer join;

right join indicates a right outer join;

full join indicates a complete external join;

The ON clause is used to specify the connection conditions.

Note: If you use the from clause to specify the inner and outer joins, you must use the on clause to specify the connection conditions.

If you use the (+) operator to specify an outer join, you must use a where clause to specify the join condition.

Inner join join

inner join

The inner join logical operator returns each line that satisfies the first (top) input join the second (bottom) input. This has the same effect as querying multiple tables with select, so internal joins are rarely used.

Another point to note is that join is inner join by default. So we can omit the inner keyword when writing an inner link.

Natural join

Natural joins look for fields with the same data type and column names in two tables, and then automatically join them and return all the results that match the criteria.

2. Outer join

The outer join returns each row that satisfies the join of the first (top) input to the second (bottom) input. It also returns any line from the first input that doesn't have a matching line in the second input. There are three types of external connections:

Left outer connection, right outer connection, all external connection. Corresponding sql: left right full outer join.

Usually we omit the outer keyword. Written: left right full join.

Both the left and right joins are based on a table, and the contents of the table are displayed in their entirety, and then the content that matches the two tables is added. If the data in the base table is not recorded in another table. Then the column appears as null in the associated result set row.

For external joins, you can also use "(+" to denote this. A few things to note about using (+):

1.(The + operator can only appear in a where clause and cannot be used in conjunction with the outer join syntax.)

2.When performing an outer join with the (+) operator, if there are multiple conditions in the where clause, the (+) operator must be included in all conditions.

For external joins, oracle can use "(+" to denote it, and 9i can use left right full outer join, which will be described below with examples.

1.Left Outer Join: Select , from employees e

left outer join departments don ( = ;

Equivalent to select , from employees e, departments dwhere ;

The result is: records of all employees and their corresponding departments, including records of employees who do not have the corresponding department ID.

2.Right Outer Join: Select from employees e

right outer join departments don ( = ;

Equivalent to select , from employees e, departments dwhere ;

The result: records for all employees and their respective departments, including records for departments without any employees.

3.full outer join: select from employees e

full outer join departments don ( = ;

The result is: records of all employees and their corresponding departments, including records of employees without department IDs and department records without any employees.

There is a database in the book, and you can feel what it means by looking at the examples in the book, but it is very troublesome to give examples here.

It is probably to put all the items in the two tables together as rows, and only one is counted for repeating, and then the items in the two tables are made as Cartesian products.

Whether the username, password, and port are correct, and whether your MySQL Server allows the user Dark to access the database from the outside

You can test it with MySQL's own client first.

mysql -u dark -p -h localhost -p 3306 and run the following command:

Then show databases, if you see the test database, run use test;

You can try it first to make sure that there is no problem with the database server configuration.

char *host = "localhost";

int port = 3306;

These two places should be written correctly. I feel that what you are above should be written by the host incorrectly.