<< SQL must know will be >> - Note 3

Create calculated fields

splice

  SELECT Concat(vend_name, ' (', vend_country, ')')
          AS ven_title
  FROM Vendors
  ORDER BY vend_name;

Compute

  SELECT prod_id, quantity, item_price,
         quantity*item_price AS expanded_price
  FROM OrderItems
  WHERE order_num = 20008;

Test calculations

SELECTCan be used to verify the statement is correct or test, for example:

    SELECT 2 * 3; -- (will return 6)
    SELECT TRIM('   abc    '); -- (will return 'abc')
    SELECT Now(); -- (will return current time)

Data handler

function

Text handling functions

  SELECT vend_name, UPPER(vend_name) AS vend_name_upcase
  FROM Vendors
  ORDER BY vend_name;

SOUNDEX()Function, a text string into any algorithm which describes a syllable represented by alphanumeric mode, may be used to look fuzzy sound

    SELECT cust_name, cust_contact
    FROM Customers
    WHERE SOUNDEX(cust_contact) = SOUNDEX('Michael Green');

Date and time handling functions

   SELECT order_num 
   FROM Orders 
   WHERE strftime('%Y', order_date) = 2012

Numerical handler

General mathematical functions ABS() COS() EXP() PI() SIN() SQRT() TAN()
are absolute value, cosine, exponential, pi, sine, square root, tangent

Aggregated data

Aggregate Functions

AVG

  SELECT AVG(prod_price) AS avg_price
  FROM Products 
  WHERE vend_id = 'DLL01';

COUNT

  SELECT COUNT(*) AS num_cust
  FROM Customers;

MAX

   SELECT MAX(prod_price) AS max_price
   FROM Products;

MIN

   SELECT MIN(prod_price) AS min_price
   FROM Products;

SUM

   SELECT SUM(quantity) AS items_ordered
   FROM OrderItems
   WHERE order_num = 20005;

Aggregation of different values

DISTINCT

   SELECT AVG(DISTINCT prod_price) AS avg_price
   FROM Products
   WHERE vend_id = 'DLL01';

A combination of aggregate functions

   SELECT COUNT(*) AS num_items,
          MIN(prod_price) AS price_min,
          MAX(prod_price) AS price_max,
          AVG(prod_price) AS price_avg
   FROM Products;

Packet data

GROUP BY

Creating groups

   SELECT vend_id, COUNT(*) AS num_prods
   FROM Products 
   GROUP BY vend_id;

GROUP BYIn the WHEREfollowing statement, ORDER BYbefore the statement

Packet filtering

WHERELine filter, HAVINGthe filter packet

   SELECT cust_id, COUNT(*) AS orders
   FROM Orders
   GROUP BY cust_id 
   HAVING COUNT(*) >= 2;

SELECT clause order

SELECT -> FROM -> WHERE -> GROUP BY -> HAVING ->ORDER BY

   SELECT vend_id, COUNT(*) AS num_prods
   FROM Products
   WHERE prod_price >= 4
   GROUP BY vend_id 
   HAVING COUNT(*) >= 2;
   ORDER BY vend_id

Using Subqueries

Subqueries

Sub-query filter

Outwardly from the process

   SELECT cust_name, cust_contact
   FROM Customers
   WHERE cust_id IN (SELECT cust_id 
                     FROM Orders 
                     WHERE order_num IN (SELECT order_num 
                                         FROM ORDERItems 
                                         WHERE prod_id = 'RGAN01'));
   

As calculated field

    SELECT cust_name, cust_state,
           (SELECT COUNT(*)
            FROM Orders 
            WHERE Orders.cust_id = Customers.cust_id) AS orders 
    FROM Customers 
    ORDER BY cust_name;

Joining tables

coupling

Creating links

    SELECT vend_name, prod_name, prod_price
    FROM Vendors, Products
    WHERE Vendors.vend_id = Products.vend_id;

Cartesian product called cross coupling, not coupled condition, directly multiplying two tables

The inner coupling

    SELECT vend_name, prod_name, prod_price 
    FROM Vendors INNER JOIN Products 
    ON Vendors.vend_id = Products.vend_id;

Inner coupling known equivalent coupler , two tables based on equality tests

Join multiple tables

    SELECT prod_name, vend_name, prod_price, quantity
    FROM OrderItems, Products, Vendors 
    WHERE Products.vend_id = Vendors.vend_id 
    AND OrderItems.prod_id = Products.prod_id 
    AND order_num = 20007;

Advanced link

Table aliases

Benefits can shorten the sentence, allowing many times the same table in a SQL statement using

    SELECT cust_name, cust_contact 
    FROM Customers AS C, Orders AS O, OrderItems AS OI 
    WHERE C.cust_id = O.cust_id 
    AND O.order_num = OI.order_num 
    AND OI.prod_id = 'RGAN01';

Self-join

    SELECT C2.cust_id, C2.cust_name, C2.cust_contact 
    FROM Customers AS C1, Customers AS C2
    WHERE C2.cust_name = C1.cust_name 
    AND C1.cust_contact = 'Jim Jones';

DBMS processing link is far faster than subqueries

Natural links

Exclude multiple times, once for each column returned

External link

    SELECT Customers.cust_id, Orders.order_num 
    FROM Customers LEFT OUTER JOIN Orders 
    ON Customers.cust_id = Orders.cust_id;

With a link aggregate function

    SELECT Customers.cust_id, COUNT(Orders.order_num) AS num_order
    FROM Customers INNER JOIN Orders 
    ON Customers.cust_id = Orders.cust_id 
    GROUP BY Customers.cust_id;

Combined Query

UNION

Use UNION

Give each SELECT statement, plus between UNION

    SELECT cust_name, cust_contact, cust_email
    FROM Customers 
    WHERE cust_state IN ('IL', 'IN', 'MI')
    UNION 
    SELECT cust_name, cust_contact, cust_email 
    FROM Customers 
    WHERE cust_name = 'Fun4All';

rule

Each query must contain the same column, expression, or aggregate functions, data types to be compatible

It contains duplicate rows

    SELECT cust_name, cust_contact, cust_email
    FROM Customers 
    WHERE cust_state IN ('IL', 'IN', 'MI')
    UNION ALL
    SELECT cust_name, cust_contact, cust_email 
    FROM Customers 
    WHERE cust_name = 'Fun4All';