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Note the result, in particular the order of the rows If these tables are as created in 9, there will be three customers details and two orders returned The CUSTOMER_ID values are returned in the order: 1,2,3 and 2,3 respectively
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3 Perform a union between the set of customerscustomer_id and orders customer_id values:
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select customer_id from customers union select customer_id from orders;
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Only the distinct customer_id values are returned sorted as: 1,2,3 4 This time, use UNION ALL:
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select customer_id from customers union all select customer_id from orders;
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There will be five rows, and they will not be sorted 5 An intersection will retrieve rows common to two queries:
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select customer_id from customers intersect select customer_id from orders;
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Two rows are common, and the result is sorted 6 A MINUS will remove common rows:
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select customer_id from customers minus select customer_id from orders;
The first set (1,2,3) minus (2,3) yields a single row All queries in this exercise are shown in the following illustration
13: Subqueries and Set Operators
Use a Set Operator to Combine Multiple Queries into a Single Query
Compound queries are two or more queries, linked with one or more set operators The end result is a single result set The examples that follow are based on two tables, OLD_DEPT and NEW_DEPT The table OLD_DEPT is intended to represent a table created with an earlier version of Oracle, when the only data type available for representing date and time data was DATE, the only option for numeric data was NUMBER, and character data was fixedlength CHAR The table NEW_DEPT uses the more tightly defined INTEGER numeric data type (which Oracle implements as a NUMBER of up to 38 significant digits but no decimal places), the more space-efficient VARCHAR2 for character data, and the TIMESTAMP data type, which can by default store date and time values with six decimals of precision on the seconds There are two rows in each table
PART II
The UNION ALL Operator
A UNION ALL takes two result sets and concatenates them together into a single result set The result sets come from two queries that must select the same number of columns, and the corresponding columns of the two queries (in the order in which they are specified) must be of the same data type group The columns do not have to have the same names Figure 13-2 demonstrates a UNION ALL operation from two tables The UNION ALL of the two tables converts all the values to the higher level of precision: the dates are returned as timestamps (the less precise DATEs padded with zeros), the character data is the more efficient VARCHAR2 with the length of the longer input column, and the numbers (though this is not obvious due to the nature of the data) will accept decimals The order of the rows is the rows from the first table in whatever order they happen to be stored followed by the rows from the second table in whatever order they happen to be stored
Figure 13-2 A UNION ALL with data type conversions
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EXAM TIP A UNION ALL will return rows grouped from each query in their natural order This behavior can be modified by placing a single ORDER BY clause at the end
The UNION Operator
A UNION performs a UNION ALL and then sorts the result across all the columns and removes duplicates The first query in Figure 13-3 returns all four rows because there are no duplicates However, the rows are now in order It may appear that the first two rows are not in order because of the values in DATED, but they are: the DNAME in the table OLD_DEPTS is 20 bytes long (padded with spaces), whereas the DNAME in NEW_DEPTS, where it is a VARCHAR2, is only as long as the name itself The spaces give the row from OLD_DEPT a higher sort value, even though the date value is less The second query in Figure 13-3 removes any leading or trailing spaces from the DNAME columns and chops off the time elements from DATED and STARTED Two of the rows thus become identical, and so only one appears in the output Because of the sort, the order of the queries in a UNION compound query makes no difference to the order of the rows returned TIP If, as a developer, you know that there can be no duplicates between two tables, then always use UNION ALL It saves the database from doing a lot of sortingYour DBA will not be pleased with you if you use UNION unnecessarily
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