The UNION [ALL], INTERSECT, MINUS Operators

See Also:

■LEVEL on page 2-86 for a discussion of how the LEVEL pseudocolumn operates in a hierarchical query

■SYS_CONNECT_BY_PATH on page 6-155 for information on retrieving the path of column values from root to node

■order_by_clause on page 18-25 for more information on the SIBLINGS keyword of ORDER BY clauses

The UNION [ALL], INTERSECT, MINUS Operators

You can combine multiple queries using the set operators UNION, UNION ALL, INTERSECT, and MINUS. All set operators have equal precedence. If a SQL statement contains multiple set operators, then Oracle evaluates them from the left to right if no parentheses explicitly specify another order.

The corresponding expressions in the select lists of the component queries of a compound query must match in number and datatype. If component queries select character data, then the datatype of the return values are determined as follows:

■If both queries select values of datatype CHAR, then the returned values have datatype CHAR.

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The UNION [ALL], INTERSECT, MINUS Operators

■If either or both of the queries select values of datatype VARCHAR2, then the returned values have datatype VARCHAR2.

Restrictions on Set Operators

■The set operators are not valid on columns of type BLOB, CLOB, BFILE, VARRAY, or nested table.

■The UNION, INTERSECT, and MINUS operators are not valid on LONG columns.

■If the select list preceding the set operator contains an expression, then you must provide a column alias for the expression in order to refer to it in the order_by_clause.

■You cannot also specify the for_update_clause with these set operators.

■You cannot specify the order_by_clause in the subquery of these operators.

■You cannot use these operators in SELECT statements containing TABLE collection expressions.

Note: To comply with emerging SQL standards, a future release of Oracle will give the INTERSECT operator greater precedence than the other set operators. Therefore, you should use parentheses to specify order of evaluation in queries that use the INTERSECT operator with other set operators.

The following examples combine the two query results with each of the set operators.

UNION Example The following statement combines the results with the UNION operator, which eliminates duplicate selected rows. This statement shows that you must match datatype (using the TO_CHAR function) when columns do not exist in one or the other table:

SELECT location_id, department_name "Department", TO_CHAR(NULL) "Warehouse" FROM departments UNION

SELECT location_id, TO_CHAR(NULL) "Department", warehouse_name FROM warehouses;

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UNION ALL Example The UNION operator returns only distinct rows that appear in either result, while the UNION ALL operator returns all rows. The UNION ALL operator does not eliminate duplicate selected rows:

SELECT product_id FROM order_items

UNION

SELECT product_id FROM inventories;

SELECT location_id FROM locations

UNION ALL

SELECT location_id FROM departments;

A location_id value that appears multiple times in either or both queries (such as ’1700’) is returned only once by the UNION operator, but multiple times by the UNION ALL operator.

INTERSECT Example The following statement combines the results with the INTERSECT operator, which returns only those rows returned by both queries:

SELECT product_id FROM inventories

INTERSECT

SELECT product_id FROM order_items;

MINUS Example The following statement combines results with the MINUS operator, which returns only rows returned by the first query but not by the second:

SELECT product_id FROM inventories

MINUS

SELECT product_id FROM order_items;

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Sorting Query Results

Sorting Query Results

Use the ORDER BY clause to order the rows selected by a query. Sorting by position is useful in the following cases:

■To order by a lengthy select list expression, you can specify its position, rather than duplicate the entire expression, in the ORDER BY clause.

■For compound queries (containing set operators UNION, INTERSECT, MINUS, or UNION ALL), the ORDER BY clause must use positions, rather than explicit expressions. Also, the ORDER BY clause can appear only in the last component query. The ORDER BY clause orders all rows returned by the entire compound query.

The mechanism by which Oracle sorts values for the ORDER BY clause is specified either explicitly by the NLS_SORT initialization parameter or implicitly by the NLS_ LANGUAGE initialization parameter. You can change the sort mechanism dynamically from one linguistic sort sequence to another using the ALTER SESSION statement. You can also specify a specific sort sequence for a single query by using the NLSSORT function with the NLS_SORT parameter in the ORDER BY clause.

See Also: Oracle9i Database Globalization Support Guide for information on the NLS parameters

Joins

A join is a query that combines rows from two or more tables, views, or materialized views. Oracle performs a join whenever multiple tables appear in the query’s FROM clause. The query’s select list 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 Conditions

Most join queries contain WHERE clause conditions that compare two columns, each from a different table. Such a condition is called a join condition. To execute a join, Oracle 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

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Joins

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, in the case of the cost-based optimization approach, statistics for the tables.

In addition to join conditions, the WHERE clause of a join query 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.

Note: You cannot specify LOB columns in the WHERE clause if the WHERE clause contains any joins. The use of LOBs in WHERE clauses is also subject to other restrictions. See Oracle9i Application Developer’s Guide - Large Objects (LOBs) for more information.

Equijoins

An 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.

See Also: "Using Join Queries: Examples" on page 18-34

Self Joins

A 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 combines and returns rows of the table that satisfy the join condition.

See Also: "Using Self Joins: Example" on page 18-36

Cartesian Products

If two tables in a join query have no join condition, then Oracle 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

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Joins

specific pair, then the optimizer may choose a join order that avoids producing an intermediate Cartesian product.

Inner Joins

An 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 Joins

An 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.

■To write a query that performs an outer join of tables A and B and returns all rows from A (a left outer join), use the LEFT [OUTER] JOIN syntax in the FROM clause, or apply the outer join operator (+) to all columns of B in the join condition in the WHERE clause. For all rows in A that have no matching rows in B, Oracle returns null for any select list expressions containing columns of B.

■To write a query that performs an outer join of tables A and B and returns all rows from B (a right outer join), use the RIGHT [OUTER] JOIN syntax in the FROM clause, or apply the outer join operator (+) to all columns of A in the join condition in the WHERE clause. For all rows in B that have no matching rows in A, Oracle returns null for any select list expressions containing columns of A.

■To write a query that performs an outer join and returns all rows from A and B, extended with nulls if they do not satisfy the join condition (a full outer join), use the FULL [OUTER] JOIN syntax in the FROM clause.

Oracle Corporation 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 join syntax:

■You cannot specify the (+) operator in a query block that also contains FROM clause join syntax.

■The (+) operator can appear only in the WHERE clause or, in the context of left-correlation (that is, when specifying the TABLE clause) in the FROM clause, and can be applied only to a column of a table or view.

■If A and B are joined by multiple join conditions, then you must use the (+) operator in all of these conditions. If you do not, then Oracle will return only the rows resulting from a simple join, but without a warning or error to advise you that you do not have the results of an outer join.

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