SQL Intermediate Workshop Authored by Jay Mussan-Levy.

SQL Intermediate Workshop

Authored by Jay Mussan-Levy

Introduction to QueryingThe power of SQL lies not in the creation of

tables, but the ability to attain information

from them.

Querying is asking the database if it has

certain things, then the database gives you the

results.

Select StatementThe way to query in SQL is through the select statement The select statement has five main clauses but “from”

is the only required ones Here is the syntax for a select statement:

select all|distinct column1, column2from table1, table2where "some condition"group by "some condition"having "some condition"order by "some condition"

Select Statement cont’dHere is an example of a simple query:

select quantity, max(price)from items_orderedgroup by quantity;

This gets displays the quantity for the most costly item for every quantity number, and its corresponding price. Groups by quantity (staring with 1)

Select Statement cont’dHere is what the results would look like:

1 1250.00

2 88.70

3 14.75

4 125.00

Select Statement cont’d Comparison Operators= equals

> greater than

< less than

>= greater than or equal to

<= less than or equal to

<> or != not equal to

LIKE string comparison test

Select Statement cont’dExample:

select first, last, title

from employee

where title LIKE 'Pro%'; Now the previous statement gets the first

name, last name, and title columns for all employees whose title begins with Pro

Select Statement cont’d

Before After first last title age salary

Jonie Weber Secretary 28 19,500.00

Potsy Weber Programmer 32 45,300.00

Dirk Smith Programmer II 45 75,020.00

John Doe Programmer III 58 100,000.00

Joe Shmoe Manager 36 62,000.00

Jane Doe Analyst 22 45,000.00

first last title

Potsy Weber Programmer

Dirk Smith Programmer II

John Doe Programmer III

Select Statement All and DistinctALL and DISTINCT will select either all or

unique records

ex)

Select DISTINCT age

from employee_info;

This will return all the unique ages you have

in the table

Select Statement All and Distinct cont’dBefore Afterfirst last id age city state

John Jones 99980 45 Payson Arizona

Mary Jones 99982 25 Payson Arizona

Eric Edwards 88232 32 San Diego California

Mary Ann Edwards 88233 32 Phoenix Arizona

Ginger Howell 98002 42 Cottonwood Arizona

Sebastian Smith 92001 23 Gila Bend Arizona

Gus Gray 22322 35 Bagdad Arizona

Mary Ann May 32326 52 Tucson Arizona

Erica Williams 32327 60 Show Low Arizona

Leroy Brown 32380 22 Pinetop Arizona

Elroy Cleaver 32382 22 Globe Arizona

age

45

25

42

23

35

52

60

Select Statement ReviewPlease go to http://sqlcourse2.com/select2.html

Scroll to the bottom of the page and do Problems 2 and 3

Select Statement Aggregate Functions

Aggregate Functions are: Min - Returns the smallest value in a given column Max - Returns the largest value in a given column Sum - Returns the sum of the numeric values in a given

column Avg - Returns the average value of a given column Count - Returns the total number of values in a given

column Count(*) - returns the number of rows in a table

Select Statement Aggregate Functions cont’d

Aggregate values are used to manipulate numeric

values in a column.

Here is an example:

Select avg(salary)

from employee;

Select Statement Aggregate Functions cont’dBefore After first last title age salary







57,803.33

Select Statement Group ByThe 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.

Select Statement Group By cont’d

Here is the syntax:

Select "columnname", SUM("columnname2")

from "tablename"

Group By “some column name";

Select Statement Group By cont’dSelect max(salary), titlefrom employeegroup by title;

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

Select Statement Group By cont’d Before After

first last title age salary







first last title Salary

Jonie Weber Secretary 19,500.00

John Doe Programmer III 100,000.00

Joe Shmoe Manager 62,000.00

Jane Doe Analyst 45,000.00

Select Statement Group By cont’dHow do you group everything of quantity 1 together,quantity 2 together, quantity 3 together, etc. ?What if you wanted to determine the largest costitem for each grouped quantity?Here is how:

Select quantity, max(price)from items_orderedgroup by quantity

Select Statement Group By cont’dResults

quantity price

1 1250.00

2 88.70

3 14.75

4 125.00

Select Statement Having Clause Must follow the Group By clause Allows you to specify conditions on the

rows for each group Which rows should be selected will be

based on the conditions you specify

Select Statement Having Clause cont’dThe syntax for Having is:

Select "columnname", sum("columnname2")

from "tablename"

group by "columnlist"

having "condition";

Select Statement Having Clause cont’dLet's say you have an employee table containing the

employee's name, title, salary, and age. If

you would like to select the average salary for

each position,you could enter:

Select title, avg(salary)

from employee

group by title;

Select Statement Having Clause cont’dBefore After first last title age salary







title salary

Secretary 19,500.00

Programmer 73,440.00

Manager 62,000.00

Analyst 45,000.00

Select Statement Having Clause cont’dBut what if you only wanted to display the

average if their salary was over 20000?

select quantity, avg(price)

from items_ordered

group by quantity

having avg(price) > 20;

Select Statement Having Clause cont’dResults

1 120.132500

2 45.799999

4 61.666667

Select Statement Order By Order by is sorting

through a rule you create Ascending descending

Syntax:

Select "columnname", Sum("columnname2")

from "tablename"

order by "rule";

Select Statement Order ByExample:Select first, last, id, age, statefrom employee_infowhere state = ‘Arizona’order by last;This statement will select 5 columns, where the State is Arizona. It will sort them by last name in ascending order. If you wanted it to be descending, change the last line to:

order by last desc;

Select Statement Order By cont’d Before After

first last id age city state























Select Statements Combining Conditions cont’d

The AND operator – Used to join 2 conditions in the WHERE clause. Both sides of the AND condition must be true for the

condition to be met and for those rows to be displayed

The OR operator – Used to join two conditions in the WHERE clause. Either side of the OR operator can be true and the

condition will be met or both sides can be true

Select Statements Combining Conditions and Boolean Operations

Syntax:

Select "columnname", sum("columnname2")

from "tablename"

where "condition1" and "condition2";


example:

Select first, last, id, age, city, state

from employee_info

where (age >= 30) and (state = ‘Arizona');


Before Afterfirst last id age city state





















Example:

Select first, last, id, age, city, state

from employee_info

where (age >= 30) or (state = ‘Arizona');

























Before After

Select Statement In and Between Conditional Operators

The In Operator can be thought of as an or Statement. This is why:where lastname IN('Hernandez', 'Jones', 'Roberts', 'Ruiz'); is the same aswhere lastname = (‘Hernandez’) or lastname = (‘Jones’) or lastname = (‘Roberts’) or lastname = ‘Ruiz’);You can also use not in to exclude people

Select Statement In and Between Conditional Operators cont’dThe between operator is used to find something within a range of numbers Here is an example:

Select employeeid, age, lastname, salaryfrom employee_infowhere age Between 30 and 40;This is the same aswhere age >= 30 and age <= 40;You can also use not between

Select Statements Mathematical FunctionsThe Mathematical Operators are:

+ addition

- subtraction* multiplication

/ division% modulos (remainder)

Select Statements Mathematical Functions cont’d

ABS(x) absolute value of x SIGN(x) -1, 0, 1, returns positive,

negative, zero MOD(x,y) returns the remainder of

x/y (same as x%y) FLOOR(x) returns the largest integer

less than or equal to x CEILING(x) or CIEL(x) returns the smallest

Integer that is >= x

Select Statements Mathematical Functions cont’d POWER(x,y) raise x to the y power ROUND(x) returns x rounded to the

nearest integer ROUND(x,d) returns the value of x

rounded to the number of decimal places specified by d

SQRT(x) returns the square-root value of x

Select Statements Mathematical Functions cont’d

Here is an example:

select round(salary), first, last

from employee;

This means that everyone’s salary will be

rounded to the nearest integer

Select Statements Mathematical Functions cont’dBefore After first last title age salary







first last salary

Jonie Weber 19,500.00

Potsy Weber 45,300.00

Dirk Smith 75,020.00

John Doe 100,000.00

Joe Shmoe 62,000.00

Jane Doe 45,000.00

Select Statement Table JoinsAll the querying you have seen so far has

been for one table, Joins will break the

barriers holding you back

Joining allows you to relate multiple tables to

each other which is the reason the databases

you are using are called relational.

Select Statements Table Joins cont’dHere is the BASIC syntax for a join:

Select "columnname"

from "tablename", "tablename2"

where "condition"

Select Statements Table Joins cont’dJoins 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

Select Statements Table Joins cont’d

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:

Select Statements Table Joins cont’dNow, 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!

Select Statements Table Joins cont’dNormalization - Data Normalization is a

database design technique that is used to get the tables in your database into at least the third normal form (3NF). Basically, this means that you want to eliminate the redundancy of non-key data when constructing your tables. Each table should only have columns that depend on the primary key.

Select Statements Table Joins cont’dNotice 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, purchasesWHERE 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.


example:SELECT employee_info.employeeid, employee_info.lastname, employee_sales.comissionFROM employee_info, employee_salesWHERE 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.

SQL Intermediate Workshop Authored by Jay Mussan-Levy.

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Transcript of SQL Intermediate Workshop Authored by Jay Mussan-Levy.