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A Quick Tour of SQL
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SELECT NAME, SALES, QUOTA, (SALES - QUOTA) FROM SALESREPS WHERE SALES < QUOTA NAME SALES QUOTA -------------- ------------ -----------Bob Smith $142,594.00 $200,000.00 Nancy Angelli $186,042.00 $300,000.00 (SALES-QUOTA) -------------$57,406.00 -$113,958.00
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AN OVERVIEW OF SQL
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You can use the same technique to list large orders in the database and find out which customer placed the order, which product was ordered, and in what quantity. You can also ask SQL to sort the orders based on the order amount:
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SELECT FROM WHERE ORDER ORDER_NUM, CUST, PRODUCT, QTY, AMOUNT ORDERS AMOUNT > 25000.00 BY AMOUNT
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ORDER_NUM CUST PRODUCT QTY AMOUNT ---------- ----- -------- ---- ----------112987 2103 4100Y 11 $27,500.00 113069 2109 775C 22 $31,350.00 112961 2117 2A44L 7 $31,500.00 113045 2112 2A44R 10 $45,000.00
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SQL not only retrieves individual pieces of data from the database, but it can be used to summarize the database contents as well. What s the average size of an order in the database This request asks SQL to look at all the orders and find the average amount:
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SELECT AVG(AMOUNT) FROM ORDERS AVG(AMOUNT) -----------$8,256.37
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SQL: The Complete Reference
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You could also ask for the average order size for a particular customer:
SELECT AVG(AMOUNT) FROM ORDERS WHERE CUST = 2103 AVG(AMOUNT) ----------$8,895.50
Finally, let s find out the total value of the orders placed by each customer. To do this, you can ask SQL to group the orders together by customer number and then total the orders for each customer:
SELECT CUST, SUM(AMOUNT) FROM ORDERS GROUP BY CUST CUST SUM(AMOUNT) ----- -----------2101 $1,458.00 2102 $3,978.00 2103 $35,582.00 2106 $4,026.00 2107 $23,132.00 2108 $7,255.00 2109 $31,350.00 2111 $6,445.00 2112 $47,925.00 2113 $22,500.00 2114 $22,100.00 2117 $31,500.00 2118 $3,608.00 2120 $3,750.00 2124 $3,082.00
Adding Data to the Database
You can also use SQL to add new data to the database. For example, suppose you just opened a new Western region sales office in Dallas, with target sales of $275,000. Here s the INSERT statement that adds the new office to the database, as office number 23:
2:
A Quick Tour of SQL
INSERT INTO OFFICES (CITY, REGION, TARGET, SALES, OFFICE) VALUES ('Dallas', 'Western', 275000.00, 0.00, 23) 1 row inserted.
AN OVERVIEW OF SQL
Similarly, if Mary Jones (employee number 109) signs up a new customer, Acme Industries, this INSERT statement adds the customer to the database as customer number 2125 with a $25,000 credit limit:
INSERT INTO CUSTOMERS (COMPANY, CUST_REP, CUST_NUM, CREDIT_LIMIT) VALUES ('Acme Industries', 109, 2125, 25000.00) 1 row inserted.
Deleting Data
Just like the SQL INSERT statement adds new data to the database, the SQL DELETE statement removes data from the database. If Acme Industries decides a few days later to switch to a competitor, you can delete Acme s customer information from the database with this statement:
DELETE FROM CUSTOMERS WHERE COMPANY = 'Acme Industries' 1 row deleted.
And if you decide to terminate all salespeople whose sales are less than their quotas, you can remove them from the database with this DELETE statement:
DELETE FROM SALESREPS WHERE SALES < QUOTA 2 rows deleted.
Updating the Database
You can also use SQL to modify data that is already stored in the database. For example, to increase the credit limit for First Corp. to $75,000, you would use the SQL UPDATE statement:
UPDATE CUSTOMERS SET CREDIT_LIMIT = 75000.00 WHERE COMPANY = 'First Corp.' 1 row updated.
SQL: The Complete Reference
The UPDATE statement can also make many changes in the database at once. For example, this UPDATE statement raises the quota for all salespeople by $15,000:
UPDATE SALESREPS SET QUOTA = QUOTA + 15000.00 8 rows updated.
Protecting Data
An important role of a database is to protect the stored data from access by unauthorized users. For example, suppose your assistant, named Mary, has not been previously authorized to insert data about new customers into the database. This SQL statement grants her that permission:
GRANT INSERT ON CUSTOMERS TO MARY Privilege granted.
Similarly, the following SQL statement gives Mary permission to update data about customers and to retrieve customer data with the SELECT statement:
GRANT UPDATE, SELECT ON CUSTOMERS TO MARY Privilege granted.
If Mary is no longer allowed to add new customers to the database, this REVOKE statement will disallow it:
REVOKE INSERT ON CUSTOMERS FROM MARY Privilege revoked.
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A Quick Tour of SQL
Similarly, this REVOKE statement will revoke all of Mary s privileges to access customer data in any way:
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