Neither of these has any matching rows in the other respective table: 'Williams' has no associated department, and no employee has the department ID 35 ('Marketing'). The employee 'Williams' and the department 'Marketing' do not appear in the query execution results. Thus the result of the execution of the query above will be: Where the DepartmentID does not match, no result row is generated. the join-predicate is satisfied), the query will combine the LastName, DepartmentID and DepartmentName columns from the two tables into a result row. Where the DepartmentID of these tables match (i.e. The queries given in the examples above will join the Employee and Department tables using the DepartmentID column of both tables. DepartmentName FROM employee, department WHERE employee. The 'explicit join notation' uses the JOIN keyword, optionally preceded by the INNER keyword, to specify the table to join, and the ON keyword to specify the predicates for the join, as in the following example: The 'implicit join notation' is no longer considered a best practice, although database systems still support it. SQL specifies two different syntactical ways to express joins: the 'explicit join notation' and the 'implicit join notation'.

Actual SQL implementations normally use other approaches, such as hash joins or sort-merge joins, since computing the Cartesian product is slower and would often require a prohibitively large amount of memory to store. The result of the join can be defined as the outcome of first taking the Cartesian product (or Cross join) of all rows in the tables (combining every row in table A with every row in table B) and then returning all rows that satisfy the join predicate. When the join-predicate is satisfied by matching non- NULL values, column values for each matched pair of rows of A and B are combined into a result row. The query compares each row of A with each row of B to find all pairs of rows that satisfy the join-predicate. Inner join creates a new result table by combining column values of two tables (A and B) based upon the join-predicate. Inner join Īn inner join requires each row in the two joined tables to have matching column values, and is a commonly used join operation in applications but should not be assumed to be the best choice in all situations. Normal uses are for checking the server's performance. In the SQL:2011 standard, cross joins are part of the optional F401, 'Extended joined table', package. The results of a CROSS JOIN can be filtered using a WHERE clause, which may then produce the equivalent of an inner join. SELECT * FROM employee INNER JOIN department ON 1 = 1 ĬROSS JOIN does not itself apply any predicate to filter rows from the joined table. In other words, it will produce rows which combine each row from the first table with each row from the second table.

CREATE TABLE department ( DepartmentID INT PRIMARY KEY NOT NULL, DepartmentName VARCHAR ( 20 ) ) CREATE TABLE employee ( LastName VARCHAR ( 20 ), DepartmentID INT REFERENCES department ( DepartmentID ) ) INSERT INTO department VALUES ( 31, 'Sales' ), ( 33, 'Engineering' ), ( 34, 'Clerical' ), ( 35, 'Marketing' ) INSERT INTO employee VALUES ( 'Rafferty', 31 ), ( 'Jones', 33 ), ( 'Heisenberg', 33 ), ( 'Robinson', 34 ), ( 'Smith', 34 ), ( 'Williams', NULL ) Cross join ĬROSS JOIN returns the Cartesian product of rows from tables in the join.

Database definition