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Data to be inserted into a database is often downloaded from another computer system or collected from other sites and stored in a sequential file. To load the data into a table, you could write a program with a loop that reads each record of the file and uses the single-row INSERT statement to add the row to the table. However, the overhead of having the DBMS repeatedly execute single-row INSERT statements may be quite high. If inserting a single row takes half a second under a typical system load, that is probably acceptable performance for an interactive program. But that performance quickly becomes unacceptable when applied to the task of bulk loading 50,000 rows of data. In this case, loading the data would require over six hours. For this reason, most commercial DBMS products include a bulk load feature that loads data from a file into a table at high speed. The ANSI/ISO SQL standard does not address this function, and it is usually provided as a stand-alone utility program rather
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than as part of the SQL language. Each vendor s utility provides a slightly different set of features, functions, and commands. When SQL is used from within an application program, another technique is frequently provided for more efficiently inserting a large amount of data into a database. The standard programmatic INSERT statement inserts a single row of data, just like the interactive single-row INSERT statements in the preceding examples. But many commercial DBMS products allow data from two or more rows (often up to hundreds of rows) to be supplied as part of a single bulk INSERT statement. All of the supplied data must be for new rows of the single table that is the target of the INSERT statement, and named in the INTO clause. Executing a bulk INSERT statement for 100 rows of data has exactly the same effect as executing 100 individual single-row INSERT statements. However, it is usually much more efficient, because it involves only one call to the DBMS. Efficiency gains from 20 percent to 30 percent and up to 300 percent or more times over single-row INSERT statements are common, depending on the DBMS brand and the particular kind of data being inserted.
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A row of data is typically deleted from a database when the entity represented by the row disappears from the outside world. For example, in the sample database: I When a customer cancels an order, the corresponding row of the ORDERS table must be deleted. I When a salesperson leaves the company, the corresponding row of the SALESREPS table must be deleted. I When a sales office is closed, the corresponding row of the OFFICES table must be deleted. If the salespeople in the office are terminated, their rows should be deleted from the SALESREPS table as well. If they are reassigned, their REP_ OFFICE columns must be updated. In each case, the row is deleted to maintain the database as an accurate model of the real world. The smallest unit of data that can be deleted from a relational database is a single row.
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The DELETE Statement
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The DELETE statement, shown in Figure 10-5, removes selected rows of data from a single table. The FROM clause specifies the target table containing the rows. The WHERE clause specifies which rows of the table are to be deleted. Suppose Henry Jacobsen, the new salesperson hired earlier in this chapter, has just decided to leave the company. The DELETE statement that removes his row from the SALESREPS table is shown next.
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SQL: The Complete Reference
Figure 10-5.
DELETE statement syntax diagram
Remove Henry Jacobsen from the database.
DELETE FROM SALESREPS WHERE NAME = 'Henry Jacobsen' 1 row deleted.
The WHERE clause in this example identifies a single row of the SALESREPS table, which SQL removes from the table. The WHERE clause should have a familiar appearance it s exactly the same WHERE clause that you would specify in a SELECT statement to retrieve the same row from the table. The search conditions that can be specified in the WHERE clause of the DELETE statement are the same ones available in the WHERE clause of the SELECT statement, as described in s 6 and 9. Recall that search conditions in the WHERE clause of a SELECT statement can specify a single row or an entire set of rows, depending on the specific search condition. The same is true of the WHERE clause in a DELETE statement. Suppose, for example, that Mr. Jacobsen s customer, InterCorp (customer number 2126) has called to cancel all its orders. Here is the DELETE statement that removes the orders from the ORDERS table: Remove all orders for InterCorp (customer number 2126).
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