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The Multirow INSERT Statement
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The second form of the INSERT statement, shown in Figure 10-3, adds multiple rows of data to its target table. In this form of the INSERT statement, the data values for the new rows are not explicitly specified within the statement text. Instead, the source of new rows is a database query, specified in the statement.
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Figure 10-3.
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Multirow INSERT statement syntax diagram
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SQL: The Complete Reference
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Adding rows whose values come from within the database itself may seem strange at first, but it s very useful in some special situations. For example, suppose you want to copy the order number, date, and amount of all orders placed before January 1, 1990, from the ORDERS table into another table, called OLDORDERS. The multirow INSERT statement provides a concise, efficient way to copy the data: Copy old orders into the OLDORDERS table.
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INSERT INTO SELECT FROM WHERE OLDORDERS (ORDER_NUM, ORDER_DATE, AMOUNT) ORDER_NUM, ORDER_DATE, AMOUNT ORDERS ORDER_DATE < '01-JAN-90'
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9 rows inserted.
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This INSERT statement looks complicated, but it s really very simple. The statement identifies the table to receive the new rows (OLDORDERS) and the columns to receive the data, just like the single-row INSERT statement. The remainder of the statement is a query that retrieves data from the ORDERS table. Figure 10-4 graphically illustrates the
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Figure 10-4.
Inserting multiple rows
10:
Database Updates
operation of this INSERT statement. Conceptually, SQL first performs the query against the ORDERS table and then inserts the query results, row by row, into the OLDORDERS table. Here s another situation where you could use the multirow INSERT statement. Suppose you want to analyze customer buying patterns by looking at which customers and salespeople are responsible for big orders those over $15,000. The queries that you will be running will combine data from the CUSTOMERS, SALESREPS, and ORDERS tables. These three-table queries will execute fairly quickly on the small sample database, but in a real corporate database with many thousands of rows, they would take a long time. Rather than running many long, three-table queries, you could create a new table named BIGORDERS to contain the required data, defined as follows:
Column
AMOUNT COMPANY NAME PERF MFR PRODUCT QTY
Information
Order amount (from ORDERS) Customer name (from CUSTOMERS) Salesperson name (from SALESREPS) Amount over/under quota (calculated from SALESREPS)
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Manufacturer ID (from ORDERS) Product ID (from ORDERS) Quantity ordered (from ORDERS)
Once you have created the BIGORDERS table, this multirow INSERT statement can be used to populate it: Load data into the BIGORDERS table for analysis.
INSERT INTO SELECT FROM WHERE AND AND BIGORDERS (AMOUNT, COMPANY, NAME, PERF, PRODUCT, MFR, QTY) AMOUNT, COMPANY, NAME, (SALES - QUOTA), PRODUCT, MFR, QTY ORDERS, CUSTOMERS, SALESREPS CUST = CUST_NUM REP = EMPL_NUM AMOUNT > 15000.00
6 rows inserted.
In a large database, this INSERT statement may take a while to execute because it involves a three-table query. When the statement is complete, the data in the BIGORDERS table will duplicate information in other tables. In addition, the BIGORDERS table won t be automatically kept up to date when new orders are added to the database, so its data may quickly become outdated. Each of these factors seems like a disadvantage. However, the subsequent data analysis queries against the BIGORDERS table can be expressed very simply they become single-table queries.
SQL: The Complete Reference
Furthermore, each of those queries will run much faster than if it were a threetable join. Consequently, this is probably a good strategy for performing the analysis, especially if the three original tables are large. In this situation, it s likely that the BIGORDERS table will be used as a temporary table for doing the analysis. It will be created and populated with data, representing a snapshot of the order status in time, the analysis programs will be run, and then the table will be emptied or dropped. The SQL1 standard specifies several logical restrictions on the query that appears within the multirow INSERT statement: I The query cannot contain an ORDER BY clause. It s useless to sort the query results anyway, because they re being inserted into a table that is, like all tables, unordered. I The query results must contain the same number of columns as the column list in the INSERT statement (or the entire target table, if the column list is omitted), and the data types must be compatible, column by column. I The query cannot be the UNION of several different SELECT statements. Only a single SELECT statement may be specified. I The target table of the INSERT statement cannot appear in the FROM clause of the query or any subqueries that it contains. This prohibits inserting part of a table into itself. The first two restrictions are structural, but the latter two were included in the standard simply to avoid complexity. As a result, these restrictions were relaxed in the SQL2 standard. The standard now allows UNION and JOIN operations and expressions in the query, basically allowing the results of a general database query to be retrieved and then inserted into a table with the INSERT statement. It also allows various forms of self-insertion, where the source table for the data to be inserted and the destination table are the same.
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