VI.1.1 SQL Tutorial

What is SQL?

SQL (pronounced "ess-que-el") stands for Structured Query Language. SQL is used to communicate with a database. According to ANSI (American National Standards Institute), it is the standard language for relational database management systems. SQL statements are used to perform tasks such as update data on a database, or retrieve data from a database. Some common relational database management systems that use SQL are: Oracle, Sybase, Microsoft SQL Server, Access, Ingres, etc. Although most database systems use SQL, most of them also have their own additional proprietary extensions that are usually only used on their system. However, the standard SQL commands such as "Select", "Insert", "Update", "Delete", "Create", and "Drop" can be used to accomplish almost everything that one needs to do with a database. This tutorial will provide you with the instruction on the basics of each of these commands as well as allow you to put them to practice using the SQL Interpreter.

Table Basics

A relational database system contains one or more objects called tables. The data or information for the database are stored in these tables. Tables are uniquely identified by their names and are comprised of columns and rows. Columns contain the column name, data type, and any other attributes for the column. Rows contain the records or data for the columns. Here is a sample table called "weather".

city, state, high, and low are the columns. The rows contain the data for this table:

Selecting Data

The select statement is used to query the database and retrieve selected data that match the criteria that you specify. Here is the format of a simple select statement:

select "column1"
  [,"column2",etc] 
  from "tablename"
  [where "condition"];
  [] = optional

The column names that follow the select keyword determine which columns will be returned in the results. You can select as many column names that you'd like, or you can use a "*" to select all columns.

The table name that follows the keyword from specifies the table that will be queried to retrieve the desired results.

The where clause (optional) specifies which data values or rows will be returned or displayed, based on the criteria described after the keyword where.

Conditional selections used in the where clause:

The LIKE pattern matching operator can also be used in the conditional selection of the where clause. Like is a very powerful operator that allows you to select only rows that are "like" what you specify. The percent sign "%" can be used as a wild card to match any possible character that might appear before or after the characters specified. For example:

select first, last, city
   from empinfo
   where first LIKE 'Er%';

This SQL statement will match any first names that start with 'Er'. Strings must be in single quotes.

Or you can specify,

select first, last
   from empinfo
   where last LIKE '%s';

This statement will match any last names that end in a 's'.

select * from empinfo
   where first = 'Eric';

This will only select rows where the first name equals 'Eric' exactly.

What will be the result of the following sample SELECT statements?

select first, last, city from empinfo; 

select last, city, age from empinfo 
where age > 30;

select first, last, city, state from empinfo
       where first LIKE 'J%'; 

select * from empinfo;

select first, last, from empinfo
       where last LIKE '%s';  

 select first, last, age from empinfo
       where last LIKE '%illia%'; 

select * from empinfo where first = 'Eric'; 

Exercise 1.

Enter select statements to:

1. Display the first name and age for everyone that's in the table.
2. Display the first name, last name, and city for everyone that's not from Payson.
3. Display all columns for everyone that is over 40 years old.
4. Display the first and last names for everyone whose last name ends in an "ay".
5. Display all columns for everyone whose first name equals "Mary".
6. Display all columns for everyone whose first name contains "Mary".

Answers

Exercise 2.

items_ordered

Enter select statements to:

1. From the items_ordered table, select a list of all items purchased for customerid 10449. Display the customerid, item, and price for this customer.
2. Select all columns from the items_ordered table for whoever purchased a Tent.
3. Select the customerid, order_date, and item values from the items_ordered table for any items in the item column that start with the letter "S".
4. Select the distinct items in the items_ordered table. In other words, display a listing of each of the unique items from the items_ordered table.
5. Make up your own select statements and submit them.

Answers

Aggregate Functions

Aggregate functions are used to compute against a "returned column of numeric data" from your SELECT statement. They basically summarize the results of a particular column of selected data. We are covering these here since they are required by the next topic, "GROUP BY". Although they are required for the "GROUP BY" clause, these functions can be used without the "GROUP BY" clause. For example:

SELECT AVG(salary)
FROM employee;

This statement will return a single result which contains the average value of everything returned in the salary column from the employee table.

Another example:

SELECT AVG(salary)
    FROM employee;
       WHERE title = 'Programmer';

 This statement will return the average salary for all employees whose title is equal to 'Programmer'

 Example:

Example:

SELECT Count(*)
FROM employees;

This particular statement is slightly different from the other aggregate functions since there isn't a column supplied to the count function. This statement will return the number of rows in the employees table.

Exercise 3.

1. Select the maximum price of any item ordered in the items_ordered table ( see above ). Hint: Select the maximum price only.>
2. Select the average price of all of the items ordered that were purchased in the month of Dec.
3. What are the total number of rows in the items_ordered table?
4. For all of the tents that were ordered in the items_ordered table, what is the price of the lowest tent? Hint: Your query should return the price only.

Answers

GROUP BY clause

The GROUP BY clause will gather all of the rows together that contain data in the specified column(s) and will allow aggregate functions to be performed on the one or more columns. This can best be explained by an example:

GROUP BY clause syntax:

SELECT column1, 
SUM(column2)
FROM "list-of-tables"
GROUP BY "column-list";

Let's say you would like to retrieve a list of the highest paid salaries in each dept:

SELECT max(salary), dept
FROM employee 
GROUP BY dept;

This statement will select the maximum salary for the people in each unique department. Basically, the salary for the person who makes the most in each department will be displayed. Their, salary and their department will be returned.

What if I wanted to display their lastname too?

For example, take a look at the items_ordered table. Let's say you want to group everything of quantity 1 together, everything of quantity 2 together, everything of quantity 3 together, etc. If you would like to determine what the largest cost item is for each grouped quantity (all quantity 1's, all quantity 2's, all quantity 3's, etc.), you would enter:

SELECT quantity, max(price)
FROM items_ordered
GROUP BY quantity;

Enter the statement in above, and take a look at the results to see if it returned what you were expecting. Verify that the maximum price in each Quantity Group is really the maximum price.

Exercise 4.

1. How many people are in each unique state in the customers table? Select the state and display the number of people in each. Hint: count is used to count rows in a column, sum works on numeric data only.
2. From the items_ordered table, select the item, maximum price, and minimum price for each specific item in the table. Hint: The items will need to be broken up into separate groups.
3. How many orders did each customer make? Use the items_ordered table. Select the customerid, number of orders they made, and the sum of their orders. Click the Group By answers link below if you have any problems.

Answers

HAVING clause

The HAVING clause allows you to specify conditions on the rows for each group - in other words, which rows should be selected will be based on the conditions you specify. The HAVING clause should follow the GROUP BY clause if you are going to use it.

HAVING clause syntax:

SELECT column1, 
SUM(column2)
FROM "list-of-tables"
GROUP BY "column-list"
HAVING "condition";

HAVING can best be described by example. Let's say you have an employee table containing the employee's name, department, salary, and age. If you would like to select the average salary for each employee in each department, you could enter:

SELECT dept, avg(salary)
FROM employee
GROUP BY dept;

But, let's say that you want to ONLY calculate & display the average if their salary is over 20000:

SELECT dept, avg(salary)
FROM employee
GROUP BY dept
HAVING avg(salary) > 20000;

Exercise 5.

1. How many people are in each unique state in the customers table that have more than one person in the state? Select the state and display the number of how many people are in each if it's greater than 1.
2. From the items_ordered table, select the item, maximum price, and minimum price for each specific item in the table. Only display the results if the maximum price for one of the items is greater than 190.00.
3. How many orders did each customer make? Use the items_ordered table. Select the customerid, number of orders they made, and the sum of their orders if they purchased more than 1 item.

Answers

ORDER BY clause

ORDER BY is an optional clause which will allow you to display the results of your query in a sorted order (either ascending order or descending order) based on the columns that you specify to order by. 

 ORDER BY clause syntax:

SELECT column1, SUM(column2)
FROM "list-of-tables"
ORDER BY "column-list" [ASC | DESC];

[ ] = optional

This statement will select the employee_id, dept, name, age, and salary from the employee_info table where the dept equals 'Sales' and will list the results in Ascending (default) order based on their Salary.

ASC = Ascending Order - default
DESC = Descending Order 

For example:

SELECT employee_id, dept, name, age, salary
FROM employee_info
WHERE dept = 'Sales'ORDER BY salary;

If you would like to order based on multiple columns, you must seperate the columns with commas. For example:

SELECT employee_id, dept, name, age, salary
FROM employee_info
WHERE dept = 'Sales'
ORDER BY salary, age DESC;

Exercise 6.

1. Select the lastname, firstname, and city for all customers in the customers table. Display the results in Ascending Order based on the lastname.
2. Same thing as exercise #1, but display the results in Descending order.
3. Select the item and price for all of the items in the items_ordered table that the price is greater than 10.00. Display the results in Ascending order based on the price.

Answers

Combining conditions and Boolean Operators

The AND operator can be used to join two or more conditions in the WHERE clause. Both sides of the AND condition must be true in order for the condition to be met and for those rows to be displayed.

SELECT column1, 
SUM(column2)
FROM "list-of-tables"
WHERE "condition1" AND 
"condition2";

The OR operator can be used to join two or more conditions in the WHERE clause also. However, either side of the OR operator can be true and the condition will be met - hence, the rows will be displayed. With the OR operator, either side can be true or both sides can be true.

For example:

SELECT employeeid, firstname, lastname, title, salary
FROM employee_info
WHERE salary >= 50000.00 AND title = 'Programmer';

This statement will select the employeeid, firstname, lastname, title, and salary from the employee_info table where the salary is greater than or equal to 50000.00 AND the title is equal to 'Programmer'. Both of these conditions must be true in order for the rows to be returned in the query. If either is false, then it will not be displayed.

Although they are not required, you can use paranthesis around your conditional expressions to make it easier to read:

SELECT employeeid, firstname, lastname, title, salary
FROM employee_info
WHERE (salary >= 50000.00) AND (title = 'Programmer');

Another Example:

SELECT firstname, lastname, title, salary
FROM employee_info
WHERE (title = 'Sales') OR (title = 'Programmer');

This statement will select the firstname, lastname, title, and salary from the employee_info table where the title is either equal to 'Sales' OR the title is equal to 'Programmer'.

Exercise 7.

1. Select the customerid, order_date, and item from the items_ordered table for all items unless they are 'Snow Shoes' or if they are 'Ear Muffs'. Display the rows as long as they are not either of these two items.
2. Select the item and price of all items that start with the letters 'S', 'P', or 'F'.

Answers

IN and BETWEEN Conditional Operators

SELECT col1, SUM(col2)
FROM "list-of-tables"
WHERE col3 IN 
       (list-of-values);
SELECT col1, SUM(col2)
FROM "list-of-tables"
WHERE col3 BETWEEN value1 
AND value2;

SELECT employeeid, lastname, salary
FROM employee_info
WHERE lastname IN ('Hernandez', 'Jones', 'Roberts', 'Ruiz');

SELECT employeeid, lastname, salary
FROM employee_info
WHERE lastname = 'Hernandez' OR lastname = 'Jones' 
OR lastname = 'Roberts'OR lastname = 'Ruiz';

SELECT employeeid, age, lastname, salary
FROM employee_info
WHERE age BETWEEN 30 AND 40;

SELECT employeeid, age, lastname, salary
FROM employee_info
WHERE age >= 30 AND age <= 40;

SELECT round(salary), firstname
FROM employee_info

Table Joins, a must

SELECT "list-of-columns"
FROM table1,table2
WHERE "search-condition(s)"

Joins can be explained easier by demonstrating what would happen if you worked with one table only, and didn't have the ability to use "joins". This single table database is also sometimes referred to as a "flat table". Let's say you have a one-table database that is used to keep track of all of your customers and what they purchase from your store:

Everytime a new row is inserted into the table, all columns will be be updated, thus resulting in unnecessary "redundant data". For example, every time Wolfgang Schultz purchases something, the following rows will be inserted into the table:

An ideal database would have two tables:

1. One for keeping track of your customers
2. And the other to keep track of what they purchase:

"Customer_info" table:

"Purchases" table:

Now, whenever a purchase is made from a repeating customer, the 2nd table, "Purchases" only needs to be updated! We've just eliminated useless redundant data, that is, we've just normalized this database!

Notice how each of the tables have a common "cusomer_number" column. This column, which contains the unique customer number will be used to JOIN the two tables. Using the two new tables, let's say you would like to select the customer's name, and items they've purchased. Here is an example of a join statement to accomplish this:

SELECT customer_info.firstname, customer_info.lastname, purchases.item
FROM customer_info, purchases
WHERE customer_info.customer_number = purchases.customer_number;

This particular "Join" is known as an "Inner Join" or "Equijoin". This is the most common type of "Join" that you will see or use.

Notice that each of the colums are always preceeded with the table name and a period. This isn't always required, however, it IS good practice so that you wont confuse which colums go with what tables. It is required if the name column names are the same between the two tables. I recommend preceeding all of your columns with the table names when using joins.

Note: The syntax described above will work with most Database Systems -including the one with this tutorial. However, in the event that this doesn't work with yours, please check your specific database documentation.

Although the above will probably work, here is the ANSI SQL-92 syntax specification for an Inner Join using the preceding statement above that you might want to try:

SELECT customer_info.firstname, customer_info.lastname, purchases.item
FROM customer_info INNER JOIN purchases
ON customer_info.customer_number = purchases.customer_number;

SELECT customer_info.firstname, customer_info.lastname, purchases.item
FROM customer_info INNER JOIN purchases
ON customer_info.customer_number = purchases.customer_number;

Another example:

SELECT employee_info.employeeid, employee_info.lastname, employee_sales.comission
FROM employee_info, employee_sales
WHERE employee_info.employeeid = employee_sales.employeeid; 

This statement will select the employeeid, lastname (from the employee_info table), and the comission value (from the employee_sales table) for all of the rows where the employeeid in the employee_info table matches the employeeid in the employee_sales table.

Exercises 10. 

1. Write a query using a join to determine which items were ordered by each of the customers in the customers table. Select the customerid, firstname, lastname, order_date, item, and price for everything each customer purchased in the items_ordered table.
2. Repeat exercise #1, however display the results sorted by state in descending order.

Creating Tables

The create table statement is used to create a new table. Here is the format of a simple create table statement:

create table "tablename"
("column1" "data type",
 "column2" "data type",
 "column3" "data type");

Format of create table if you were to use optional constraints:

create table "tablename"
("column1" "data type" 
         [constraint],
 "column2" "data type" 
         [constraint],
 "column3" "data type" 
        [constraint]);
 [ ] = optional

Note: You may have as many columns as you'd like, and the constraints are optional.

Example:

create table employee
(first varchar(15),
 last varchar(20),
 age number(3),
 address varchar(30),
 city varchar(20),
 state varchar(20));

To create a new table, enter the keywords create table followed by the table name, followed by an open parenthesis, followed by the first column name, followed by the data type for that column, followed by any optional constraints, and followed by a closing parenthesis. It is important to make sure you use an open parenthesis before the beginning table, and a closing parenthesis after the end of the last column definition. Make sure you seperate each column definition with a comma. All SQL statements should end with a ";".

The table and column names must start with a letter and can be followed by letters, numbers, or underscores - not to exceed a total of 30 characters in length. Do not use any SQL reserved keywords as names for tables or column names (such as "select", "create", "insert", etc).

Data types specify what the type of data can be for that particular column. If a column called "Last_Name", is to be used to hold names, then that particular column should have a "varchar" (variable-length character) data type.

Here are the most common Data types:

What are constraints? When tables are created, it is common for one or more columns to have constraints associated with them. A constraint is basically a rule associated with a column that the data entered into that column must follow. For example, a "unique" constraint specifies that no two records can have the same value in a particular column. They must all be unique. The other two most popular constraints are "not null" which specifies that a column can't be left blank, and "primary key". A "primary key" constraint defines a unique identification of each record (or row) in a table. All of these and more will be covered in the future Advanced release of this Tutorial. Constraints can be entered in this SQL interpreter, however, they are not supported in this Intro to SQL tutorial & interpreter. They will be covered and supported in the future release of the Advanced SQL tutorial - that is, if "response" is good.

It's now time for you to design and create your own table. You will use this table throughout the rest of the tutorial. If you decide to change or redesign the table, you can either drop it and recreate it or you can create a completely different one. The SQL statement drop will be covered later.

Exercise 11. 

You have just started a new company. It is time to hire some employees. You will need to create a table that will contain the following information about your new employees: firstname, lastname, title, age, and salary. 

Answer

Inserting into a Table

The insert statement is used to insert or add a row of data into the table.

To insert records into a table, enter the key words insert into followed by the table name, followed by an open parenthesis, followed by a list of column names separated by commas, followed by a closing parenthesis, followed by the keyword values, followed by the list of values enclosed in parenthesis. The values that you enter will be held in the rows and they will match up with the column names that you specify. Strings should be enclosed in single quotes, and numbers should not.

insert into "tablename"
 (first_column,...last_column)
  values (first_value,...last_value);

In the example below, the column name first will match up with the value 'Luke', and the column name state will match up with the value 'Georgia'.

Example:

insert into employee
  (first, last, age, address, city, state)
  values ('Luke', 'Duke', 45, '2130 Boars Nest', 
          'Hazard Co', 'Georgia');

Note: All strings should be enclosed between single quotes: 'string'

Exercise 12. 

It is time to insert data into your new employee table.

Your first three employees are the following:

Jonie Weber, Secretary, 28, 19500.00
Potsy Weber, Programmer, 32, 45300.00
Dirk Smith, Programmer II, 45, 75020.00

Enter these employees into your table first, and then insert at least 5 more of your own list of employees in the table.

After they're inserted into the table, enter select statements to:

1. Select all columns for everyone in your employee table.
2. Select all columns for everyone with a salary over 30000.
3. Select first and last names for everyone that's under 30 years old.
4. Select first name, last name, and salary for anyone with "Programmer" in their title.
5. Select all columns for everyone whose last name contains "ebe".
6. Select the first name for everyone whose first name equals "Potsy".
7. Select all columns for everyone over 80 years old.
8. Select all columns for everyone whose last name ends in "ith".

Answers

Updating Records

The update statement is used to update or change records that match a specified criteria. This is accomplished by carefully constructing a where clause.

update "tablename"
set "columnname" = 
    "newvalue"
 [,"nextcolumn" = 
   "newvalue2"...]
where "columnname" 
  OPERATOR "value" 
 [and|or "column" 
  OPERATOR "value"];

 [] = optional

 [The above example was line wrapped for better viewing on this Web page.]

Examples:

update phone_book
  set area_code = 623
  where prefix = 979;

update phone_book
  set last_name = 'Smith', prefix=555, suffix=9292
  where last_name = 'Jones';

update employee
  set age = age+1
  where first_name='Mary' and last_name='Williams';

Exercise 13.

After each update, issue a select statement to verify your changes.

1. Jonie Weber just got married to Bob Williams. She has requested that her last name be updated to Weber-Williams.
2. Dirk Smith's birthday is today, add 1 to his age.
3. All secretaries are now called "Administrative Assistant". Update all titles accordingly.
4. Everyone that's making under 30000 are to receive a 3500 a year raise.
5. Everyone that's making over 33500 are to receive a 4500 a year raise.
6. All "Programmer II" titles are now promoted to "Programmer III".
7. All "Programmer" titles are now promoted to "Programmer II".

Answers

Deleting Records

The delete statement is used to delete records or rows from the table.

delete from "tablename"

where "columnname" 
  OPERATOR "value" 
[and|or "column" 
  OPERATOR "value"];

[ ] = optional

[The above example was line wrapped for better viewing on this Web page.]

Examples:

delete from employee;

 Note: if you leave off the where clause, all records will be deleted!

delete from employee
  where lastname = 'May';

delete from employee
  where firstname = 'Mike' or firstname = 'Eric';

To delete an entire record/row from a table, enter "delete from" followed by the table name, followed by the where clause which contains the conditions to delete. If you leave off the where clause, all records will be deleted.

Exercise 14.

(Use the select statement to verify your deletes):

1. Jonie Weber-Williams just quit, remove her record from the table.
2. It's time for budget cuts. Remove all employees who are making over 70000 dollars.

Answers

Drop a Table

The drop table command is used to delete a table and all rows in the table.

To delete an entire table including all of its rows, issue the drop table command followed by the tablename. drop table is different from deleting all of the records in the table. Deleting all of the records in the table leaves the table including column and constraint information. Dropping the table removes the table definition as well as all of its rows.

drop table "tablename"

Example:

drop table myemployees_ts0211;

Exercise 15.

Drop your employee table.

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based on http://sqlcourse2.com/