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The SQL standard specifies several logical restrictions on the query that appears within the multirow INSERT statement: 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. The query results must contain the same number of columns as the column list in the INSERT statement (or as the entire target table, if the column list is omitted), and the data types must be compatible, column by column.
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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, it s 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 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 singlerow 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. When thousands of rows are loaded, the performance gain can be an order of magnitude or more.
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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: When a customer cancels an order, the corresponding row of the ORDERS table must be deleted. When a salesperson leaves the company, the corresponding row of the SALESREPS table must be deleted. 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 column values must be updated.
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FIGURE 10-5
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DELETE statement syntax diagram
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.
The DELETE Statement
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. Remove Henry Jacobsen from the database.
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