At a minimum how many join conditions should be present to avoid a Cartesian product in Self Join
A Cartesian join or Cartesian product is a join of every row of one table to every row of another table. This normally happens when no matching join columns are specified. For example, if table A with 100 rows is joined with table B with 1000 rows, a Cartesian join will return 100,000 rows. Show
Note: A Cartesian product may indicate a missing join condition. A query must have at least (N-1) join conditions to prevent a Cartesian product, where N is the number of tables in the query. However a Cartesian product may be something valid; for instance, in a star schema, a Cartesian join between dimension tables is not unusual. Syntax examples[edit]Using Oracle join syntax: SELECT * FROM emp, dept; SELECT * FROM emp, dept WHERE dept.deptno = 10 AND emp.sal > 10000;Using ANSI join syntax: SELECT * FROM emp CROSS JOIN dept;Also see[edit]
A join is a query that combines rows from two or more tables, views, or materialized views. Oracle Database performs a join whenever multiple tables appear in the FROM clause of the query. The select list of the query can select any columns from any of these tables. If any two of these tables have a column name in common, then you must qualify all references to these columns throughout the query with table names to avoid ambiguity. Join ConditionsMost join queries contain at least one join condition, either in the FROM clause or in the WHERE clause. The join condition compares two columns, each from a different table. To execute a join, Oracle Database combines pairs of rows, each containing one row from each table, for which the join condition evaluates to TRUE. The columns in the join conditions need not also appear in the select list. To execute a join of three or more tables, Oracle first joins two of the tables based on the join conditions comparing their columns and then joins the result to another table based on join conditions containing columns of the joined tables and the new table. Oracle continues this process until all tables are joined into the result. The optimizer determines the order in which Oracle joins tables based on the join conditions, indexes on the tables, and, any available statistics for the tables. A WHERE clause that contains a join condition can also contain other conditions that refer to columns of only one table. These conditions can further restrict the rows returned by the join query. EquijoinsAn equijoin is a join with a join condition containing an equality operator. An equijoin combines rows that have equivalent values for the specified columns. Depending on the internal algorithm the optimizer chooses to execute the join, the total size of the columns in the equijoin condition in a single table may be limited to the size of a data block minus some overhead. The size of a data block is specified by the initialization parameter DB_BLOCK_SIZE. Self JoinsA self join is a join of a table to itself. This table appears twice in the FROM clause and is followed by table aliases that qualify column names in the join condition. To perform a self join, Oracle Database combines and returns rows of the table that satisfy the join condition. Cartesian ProductsIf two tables in a join query have no join condition, then Oracle Database returns their Cartesian product. Oracle combines each row of one table with each row of the other. A Cartesian product always generates many rows and is rarely useful. For example, the Cartesian product of two tables, each with 100 rows, has 10,000 rows. Always include a join condition unless you specifically need a Cartesian product. If a query joins three or more tables and you do not specify a join condition for a specific pair, then the optimizer may choose a join order that avoids producing an intermediate Cartesian product. Inner JoinsAn inner join (sometimes called a simple join) is a join of two or more tables that returns only those rows that satisfy the join condition. Outer JoinsAn outer join extends the result of a simple join. An outer join returns all rows that satisfy the join condition and also returns some or all of those rows from one table for which no rows from the other satisfy the join condition.
You cannot compare a column with a subquery in the WHERE clause of any outer join, regardless which form you specify. You can use outer joins to fill gaps in sparse data. Such a join is called a partitioned outer join and is formed using the query_partition_clause of the join_clause syntax. Sparse data is data that does not have rows for all possible values of a dimension such as time or department. For example, tables of sales data typically do not have rows for products that had no sales on a given date. Filling data gaps is useful in situations where data sparsity complicates analytic computation or where some data might be missed if the sparse data is queried directly. See Also:
Oracle recommends that you use the FROM clause OUTER JOIN syntax rather than the Oracle join operator. Outer join queries that use the Oracle join operator (+) are subject to the following rules and restrictions, which do not apply to the FROM clause OUTER JOIN syntax:
If the WHERE clause contains a condition that compares a column from table B with a constant, then the (+) operator must be applied to the column so that Oracle returns the rows from table A for which it has generated nulls for this column. Otherwise Oracle returns only the results of a simple join. In a query that performs outer joins of more than two pairs of tables, a single table can be the null-generated table for only one other table. For this reason, you cannot apply the (+) operator to columns of B in the join condition for A and B and the join condition for B and C. Refer to SELECT for the syntax for an outer join. AntijoinsAn antijoin returns rows from the left side of the predicate for which there are no corresponding rows on the right side of the predicate. It returns rows that fail to match (NOT IN) the subquery on the right side. SemijoinsA semijoin returns rows that match an EXISTS subquery without duplicating rows from the left side of the predicate when multiple rows on the right side satisfy the criteria of the subquery. Semijoin and antijoin transformation cannot be done if the subquery is on an OR branch of the WHERE clause. How many join conditions should be there to avoid a Cartesian join for joining three tables?Notice that when joining three tables, you use two join conditions (linking two tables each) to achieve a natural join. When you join four tables, you use three such join conditions. In general, if you join n tables, you need n ? 1 join conditions to avoid a Cartesian product.
How do I avoid a Cartesian product in an inner join?To avoid a Cartesian product, you must specify how the tables should be combined. Typically, you want to pair rows based on matching values in one or more key columns of each table.
How many join conditions can exist for N tables?In an n-table join, the maximum number of possible combinations is n! (n factorial). For example, in a three-table join, you could potentially join the tables in six (3 * 2 * 1) combinations.
Which join is used for Cartesian product?A CROSS JOIN is a JOIN operation that produces the Cartesian product of two tables.
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