Oracle series five multi-table query

Set Descartes
Descartes rally produced under the following conditions:

Omitted connection condition
Join condition is invalid
All rows in all tables are interconnected

To avoid a Cartesian set, may be added in effective connection conditions WHERE.

Oracle is connected

using a connection query data in multiple tables.

SELECT    table1.column, table2.column
FROM    table1, table2
WHERE    table1.column1 = table2.column2;

Write connection condition in the WHERE clause.
When the same column in the table, with the table name prefix before the column name

Equijoins

Example

SELECT
    employees.employee_id,
    employees.last_name,
    employees.department_id,
    departments.department_id,
    departments.location_id
FROM
    employees,
    departments
WHERE
    employees.department_id = departments.department_id;

A plurality of connection conditions AND operator

Distinguish duplicate column names

Use table prefix to distinguish the same column in the plurality of tables.
Columns having the same column name in different tables may be identified by a table alias.

Alias table

Using aliases, simplify the query.
Use table prefix can improve the efficiency.

SELECT
    e.employee_id,
    e.last_name,
    e.department_id,
    d.department_id,
    d.location_id
FROM
    employees e,
    departments d
WHERE
    e.department_id = d.department_id;

Connecting a plurality of tables

Connecting the n-tables, at least n-1 connections conditions. For example: three connection table, at least two connection conditions.

Non-Equijoins
column salary in the EMPLOYEES table should be among the highest wages in JOB_GRADES table with the minimum wage

examples

SELECT
    e.last_name,
    e.salary,
    j.grade_level
FROM
    employees e,
    job_grades j
WHERE
    e.salary BETWEEN j.lowest_sal AND j.highest_sal;

The inner and outer joins

En: combined with two or more rows of the same column of a table, the result set does not contain a table and another table rows that do not match
External connection: two tables satisfy the join condition in addition return line also returns the left (or right) in the table row condition is not satisfied during the connection, this connection is called a left (or right) outer join. No matching row, the corresponding result table is empty (NULL). WHERE clause conditions are similar to the outer connecting the internal connection, but is connected to the back of the list does not match the condition to increase the outer connecting line operator, i.e., with the plus sign in parentheses (+).
In SQL: 1999, the connector returns only the data that satisfy the join condition
During the connection the two tables in addition to the return line further satisfy the join condition to return the left (or right) table row condition is not satisfied, this connection is called a left (or right) outer join.
During the connection the two tables in addition to the return line further satisfy the join condition in the return line condition is not satisfied two tables, this connection is called a full external connection.

Outer join syntax

Use outer join query data does not meet the join condition.
It is connected to an outer symbol (+).

右外连接
SELECT    table1.column, table2.column
FROM    table1, table2
WHERE    table1.column(+) = table2.column;
左外连接
SELECT    table1.column, table2.column
FROM    table1, table2
WHERE    table1.column = table2.column(+);

Examples

SELECT
    e.last_name,
    e.department_id,
    d.department_name
FROM
    employees e,
    departments d
WHERE
    e.department_id (+) = d.department_id;

Since the connection
Example

SELECT
    worker.last_name
    || ' works for '
    || manager.last_name
FROM
    employees worker,
    employees manager
WHERE
    worker.manager_id = manager.employee_id;

1999 Grammar connection

SELECT    table1.column, table2.column
FROM    table1
[CROSS JOIN table2] |
[NATURAL JOIN table2] |
[JOIN table2 USING (column_name)] |
[JOIN table2
  ON(table1.column_name = table2.column_name)] |
[LEFT|RIGHT|FULL OUTER JOIN table2
  ON (table1.column_name = table2.column_name)];

ON clause to create a connection

Natural join condition is listed as a connection with the same name of.
ON clause can specify additional connection conditions.
The connection condition is separate from other conditions.
ON clause makes statements higher legibility

Examples

SELECT
    e.employee_id,
    e.last_name,
    e.department_id,
    d.department_id,
    d.location_id
FROM
    employees e
    JOIN departments d ON ( e.department_id = d.department_id );

ON clause to create multi-table joins
Example

SELECT
    employee_id,
    city,
    department_name
FROM
    employees e
    JOIN departments d ON d.department_id = e.department_id
    JOIN locations l ON d.location_id = l.location_id;

Left outer
exemplary

SELECT
    e.last_name,
    e.department_id,
    d.department_name
FROM
    employees e
    LEFT OUTER JOIN departments d ON ( e.department_id = d.department_id );

Right outer join
example

SELECT
    e.last_name,
    e.department_id,
    d.department_name
FROM
    employees e
    RIGHT OUTER JOIN departments d ON ( e.department_id = d.department_id );

Full outer join

SELECT
    e.last_name,
    e.department_id,
    d.department_name
FROM
    employees e
    FULL OUTER JOIN departments d ON ( e.department_id = d.department_id );

Oracle series five multi-table query

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