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Subqueries and Query Expressions
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Using these building blocks, SQL lets you combine their table values using the following operations: JOIN SQL provides explicit support for full cross-product joins (cross joins), natural joins, inner joins, and all types of outer joins (left, right, and full), as described in 7. A JOIN operation takes two tables as its input and produces a table of combined query results according to the join specification. UNION The SQL UNION operation provides explicit support for merging the rows of two compatible tables (that is, two tables having the same number of columns and with corresponding columns having the same data types). The UNION operation takes two tables as its input and produces a single merged table of query results. EXCEPT The SQL EXCEPT operation takes two tables as its input and produces as its output a table containing the rows that appear in the first table but that do not appear in another table that is, the rows that are missing from the second table. Conceptually, the EXCEPT operation is like table subtraction. The rows of the second table are taken away from the rows of the first table, and the answer is the remaining rows of the first table. INTERSECT The SQL INTERSECT operation takes two tables as its input and produces as its output a table containing the rows that appear in both input tables.
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UNION, INTERSECT, and EXCEPT Operations
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The UNION, INTERSECT, and EXCEPT operations provide set operations for combining two input tables to form an output table. Nearly all vendors support UNION, but support for INTERSECT and EXCEPT is inconsistent across vendors. For example, Oracle uses the keyword MINUS instead of EXCEPT. All three of the operations require that the two input tables be union-compatible they must have the same number of columns, and the corresponding columns of each table must have identical data types. Here are some simple examples of SQL query expressions involving UNION, INTERSECT, and EXCEPT operations based on the sample database: Show all products for which there is an order over $30,000 or more than $30,000 worth of inventory on hand.
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(SELECT MFR, PRODUCT FROM ORDERS WHERE AMOUNT > 30000.00) UNION (SELECT MFR_ID, PRODUCT_ID FROM PRODUCTS WHERE (PRICE * QTY_ON_HAND) > 30000);
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Show all products for which there is an order over $30,000 and more than $30,000 worth of inventory on hand.
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(SELECT MFR, PRODUCT FROM ORDERS WHERE AMOUNT > 30000.00) INTERSECT (SELECT MFR_ID, PRODUCT_ID FROM PRODUCTS WHERE (PRICE * QTY_ON_HAND) > 30000);
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Show all products for which there is an order over $30,000 except for those products that sell for under $100.
(SELECT MFR, PRODUCT FROM ORDERS WHERE AMOUNT > 30000.00) EXCEPT (SELECT MFR_ID, PRODUCT_ID FROM PRODUCTS WHERE PRICE < 100.00);
By default, the UNION, INTERSECT, and EXCEPT operations eliminate duplicate rows during their processing. This is usually the desired result, as it is in these examples, but occasionally you may need to suppress the elimination of duplicate rows. You can do this by specifying the UNION ALL, INTERSECT ALL, or EXCEPT ALL forms of the operations. Note each of these examples produces a two-column table of query results. The results come from two different source tables within the database the ORDERS table and the PRODUCTS table. However, the columns selected from these tables have the same corresponding data types, so they can be combined using these operations. In the sample database, the corresponding columns have different names in the two tables. (The manufacturer-ID column is named MFR in the ORDERS table but named MFR_ID in the PRODUCTS table.)
Query Expressions in the FROM Clause
SQL query expressions provide a much more powerful and flexible method for generating and combining tables of query results than the simple subquery and UNION operations provided by the SQL1 standard. To make query expressions even more useful and more general purpose, the SQL standard now allows them to appear almost anywhere that a table reference could appear in a SQL1 query. In particular, a query expression can appear in place of a table name in the FROM clause. Here is a simple example of a query for the sample database that uses this feature: Show the names and total outstanding orders of all customers with credit limits over $50,000.
SELECT COMPANY, TOT_ORDERS FROM CUSTOMERS, (SELECT CUST, SUM(AMOUNT) AS TOT_ORDERS FROM ORDERS GROUP BY CUST) A WHERE (CREDIT_LIMIT > 50000.00) AND (CUST_NUM = CUST);
The second table name in the FROM clause of the main query is not a table name at all, but a full-blown query expression. In fact, the expression could have been much more complex, involving UNION or JOIN operations. When a query expression appears in the FROM clause, as it does here, the DBMS conceptually carries it out first, before any other processing of the query, and creates a temporary table of the query results generated by the query expression. In this case, this temporary table consists of two columns, listing each customer number and the total of orders for that customer number. This temporary table then acts as one of the source tables for the main query. In this example, its contents are joined to the CUSTOMER table to obtain the company name and generate the answer to the main question.
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