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SQL: A Beginner s Guide
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CD_NAME: VARCHAR(60) Patsy Cline: 12 Greatest Hits Out of Africa Fundamental Orlando Court and Spark
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IN_STOCK: INT 32 29 34 5 22
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Figure 8-6
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The CD_INVENTORY_2 table with five rows of data
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As you can see, no columns are specified in the INSERT INTO clause; as a result, values will be inserted into both columns in the CD_INVENTORY_2 table. In the second line of the statement, a SELECT statement is used to pull values from the CD_NAME and IN_STOCK columns of the CD_INVENTORY table. The values will then be inserted into their respective columns in the CD_INVENTORY_2 table, as shown in Figure 8-6. Notice that the CD_INVENTORY_2 table contains the same five rows of data that are shown in Figure 8-5, only the CD_INVENTORY_2 table contains only two columns: CD_NAME_2 and IN_STOCK_2. Like any other SELECT statement, the SELECT statement that you use in an INSERT statement can contain a WHERE clause. In the following INSERT statement, the SELECT statement contains a WHERE clause that limits the IN_STOCK values to an amount greater than 10:
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INSERT INTO CD_INVENTORY_2 SELECT CD_NAME, IN_STOCK FROM CD_INVENTORY WHERE IN_STOCK > 10;
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If you were to execute this statement, only four rows would be added to the CD_ INVENTORY_2 table, rather than the five rows we saw in the previous example. The WHERE clause in this case works just like the WHERE clause in any SELECT statement. As a result, any row with an IN_STOCK value that is not greater than 10 is eliminated from the query results. Those new filtered results are then inserted into the CD_INVENTORY_2 table.
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As its name implies, the UPDATE statement allows you to update data in your SQL database. With the UPDATE statement, you can modify data in one or more rows for one or more columns. The syntax for the UPDATE statement can be shown as follows: UPDATE <table name> SET <set clause expression> [ { , <set clause expression> } . . . ] [ WHERE <search condition> ]
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8:
Modifying SQL Data
As you can see, the UPDATE clause and the SET clause are required, and the WHERE clause is optional. In the UPDATE clause, you must specify the name of the table (or view) that you re updating. In the SET clause, you must specify one or more set clause expressions, which I discuss in more detail later in this chapter. In the WHERE clause, as with the WHERE clause in a SELECT statement (see 7), you must specify a search condition. The WHERE clause works here in much the same way it does in the SELECT statement. You specify a condition or set of conditions that act as a filter for the rows that are updated. Only the rows that meet these conditions are updated. In other words, only rows that evaluate to true are updated.
NOTE
SQL supports using view names in UPDATE statements. However, if the view is based on multiple tables, all columns being updated must come from a single base table, and there may be other restrictions as described in your DBMS documentation.
Now let s return to the SET clause. As you can see, the clause includes the <set clause expression> placeholder. You must specify one or more set clause expressions. If you specify more than one, you must separate them with a comma. The syntax of the <set clause expression> placeholder can be broken down as follows: <column name> = <value expression> Basically, you must specify a column name (from the table that you re updating) and provide a value that the value in the column should equal. For example, suppose you want a value in the IN_STOCK column to be changed to 37. (It doesn t matter what the current value is.) The set clause expression would be as follows: IN_STOCK = 37. In this case, the value expression is 37; however, the value expression can be more complicated than that. For example, you can base the new value on an old value: IN_STOCK = (IN_STOCK + 1). In this case, the value expression is IN_STOCK + 1, which adds the current value in the IN_STOCK column to 1 to give you a new value. In this case, if the original value was 37, the new value will be 38. Now that we ve taken a look at the various parts of the UPDATE statement, let s put it all together using some examples. The examples we ll be looking at are based on the CD_ INVENTORY table, which is shown in Figure 8-5. In the first example, I use the UPDATE statement to change the values of the IN_STOCK column to 27, as shown in the following SQL statement:
UPDATE CD_INVENTORY SET IN_STOCK = 27;
This statement does exactly what you might expect: changes every row in the CD_ INVENTORY table so that the IN_STOCK column for each row contains a value of 27. This is fine if that s what you want, but it is unlikely that you ll want to change every row in a table so that one of the column values is the same in every row. More likely than not, you ll want to qualify the update by using a WHERE clause.
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