SQL Basics

SQL Commands With Examples
Jagriti Goswami

Outline
§  SQL Overview §  Copy Rows From One Table Into another §  Ordering Results
§  Installation And Setup §  Using Aliases §  Grouping Data
§  Creating A Database §  The NULL Value §  Conditional Expressions
§  Creating A Table §  NOT NULL Constraint §  Accessing Related Tables
§  Inserting Values §  DEFAULT Constraint §  Join Multiple Tables
§  Retrieving All Rows And Columns §  UNIQUE Constraint §  String Functions
§  Retrieving Subset Of Rows §  Primary Key Constraint §  Numeric Functions
§  Retrieving Top N number Rows §  Foreign Key Constraint §  Date And Time Functions
§  Retrieving Top X percent Rows §  Changing A Schema §  Aggregating Data
§  Retrieving Subset Of Columns §  Import Files §  Concatenating Column Data
§  Counting Rows §  Saving Query Results §  Searching For Patterns
§  Updating Data §  Filtering Data: WHERE Clause §  UNION Operator
§  Deleting Data §  Filtering Data: LIKE Operator §  Transactions
§  Deleting Table §  Filtering Data: IN Operator
§  Inserting Rows §  Removing Duplicates

SQL Overview
Ø  Structured Query Language (SQL)
Ø  Creates a database and defines its structure
Ø  Performs querying database
Ø  Controls database security
Ø  SQL has two sublanguage: Data control Language (DCL) & Data Manipulating Language (DML)
Ø  DCL deals with defining database (creating new table, fields) and with database security
Ø  DML deals with queries and data manipulation
Ø  Databases that uses SQL: MS SQL Server, Oracle, MySQL, SQLite, IBM DB2, Postgre SQL, Big Data
Ø  Install Azure Data Studio
Ø  Tools used for SQL:
Ø  Azure Data Studio (Windows, Mac OS, Linux)
Ø  SQL Server Management Studio (SSMS) (Windows)
Ø  SQL Server Data Tools (SSDT) (Windows)
Ø  Visual Studio Code (T-SQL) (Windows, Mac OS, Linux)
Ø  SQL Workbench

Ø  Installation of Microsoft SQL Server 2017 on macOS Mojave version 10.14.4
1.  Download and install Docker à Create Docker account (Link)
§  Install Docker : Download Docker Community Edition for Mac. (Link).
§  This enables to run SQL Server from within a Docker container
§  To install Docker, download .dmg file à drag the Docker.app icon to Application folder
§  To launch Docker, open Docker à provide mac password .
§  Docker installs its networking components and links to the Docker apps
§  Increase the memory: from Docker icon select Preferences à change memory to 4GB (because SQL Server
needs at least 3.25GB) à click Apply & Restart
2.  Download and install SQL Server: On Terminal run à $ docker pull microsoft/mssql-server-linux
3.  Launch the Docker Image: open a Terminal window and run à $ docker run –d –name sql_server_demo –e
‘ACCEPT_EULA=Y’ –e ‘SA_PASSWORD=reallyStrongPwd123’ –p 1433:1433 microsoft/mssql-server-linux
(name, password can be changed)
4.  Check the Docker container: run à $ docker ps
5.  Install sql-cli: run $ npn install –g sql-cli OR $ sudo npn install –g sql-cli
6.  Connect to SQL server: run à $ mssql –u sa –p reallyStrongPwd123
7.  Install Azure Data Studio (Link) & Connect to SQL server: open Azure Data Studio à Welcome page à under Start
click New Connection à provide Server Name: localhost, Authentication Type: SQL Login, user name: sa,
password: reallyStrongPwd123, database name: <default>, server group: <default>
Installation and Setup

Ø  Start Azure Data Studio (see instructions on installation and setup slide)
Ø  Create a new database
§  Right-click on server, localhost ! select New Query --> run following SQL command
CREATE DATABASE database_name;
Creating a Database

Ø  Create a new Table name test in the SQl_Tutorial Database
Ø  Change the connection context from Master to SQl_Tutorial
Ø  Run the following command as shown in below figure à

Creating a Table

Ø  Inserting values into table name test in the SQl_Tutorial Database

Inserting Values

Ø  DB SQl_Tutorial, Table Customer:

Ø  Inserting values into table Customer:
Example Tables: Customer
Customer

Ø  DB SQl_Tutorial, Table item:

Ø  DB SQl_Tutorial, Table sale:
Example Tables: item, sale
item sale
Ø  Inserting values in table item:
Ø  Inserting values in table sale:

Ø  Selecting all columns and rows from table Customer

Retrieving All Columns And Rows

Ø  Selecting specific rows from table Customer

Retrieving Subset Of Rows

Ø  Retrieving top N number rows from table Disease using SELECT TOP statement followed by number of rows

Retrieving Top N number Rows

Ø  Retrieving top x percent rows from table Disease using SELECT TOP x PERCENT statement

Retrieving Top X percent Rows

Ø  Selecting specific columns and all rows from table Customer

Retrieving Subset Of Columns

Ø  Counting rows in table Customer

Counting Rows

Ø  Updating data in table Customer

Updating Data

Ø  Deleting data in table Customer

Deleting Data

Ø  Deleting existing table test_tb from SQl_Tutorial Database

Deleting Table

Ø  Inserting rows into table

Inserting Rows

Ø  Copying rows from table test into table test_tb_3 using WHERE clause-- WHERE a IN (2, 3)

Copying Rows From One Table Into Another

Ø  Retrieving rows and columns using aliases

Using Aliases

Ø  Retrieving rows with NULL value from table test_tb_2
Ø  Retrieving rows which are NOT NULL from table test_tb_2

The NULL Value

Ø  Create a new table test_tb_1 with “NOT NULL” Constraint
Ø  The insertion of a row which doesn’t have a value will fail

Constraining columns: NOT NULL

Ø  Create a new table test
Ø  Insert row values with DEFAULT constraint

Constraining columns: DEFAULT

Ø  Create a new table test
Ø  Insert row values with UNIQUE constraint
Ø  NULL value or NULL state is exempt from the UNIQUE constraint

Constraining columns: UNIQUE

Ø  An ID column is a column with unique, sequential values for each row in a table
Ø  Create ID column with auto increment integer primary key
Ø  Auto increment Primary key in SQL Server is defined using IDENTITY and PRIMARY KEY constraint

Primary Key Constraint On Create Table

Ø  Define PRIMARY KEY constraint on multiple columns when the table is created.

Multi-Column Primary Key Constraint On Create Table

Ø  Define PRIMARY KEY constraint on multiple columns on existing table.

Multi-Column Primary Key Constraint On Existing Table

Ø  Enable, Disable, and Drop a PRIMARY KEY constraint

Disable, Enable and Drop A Primary Key Constraint

Ø  A FOREIGN KEY links two tables together
Ø  Foreign key in one table (child table) refers to the Primary key in another table (referenced or parent table)
Ø  Define FOREIGN KEY constraint when the table is created.
Ø  Concept is shown below with example tables Customer, and Orders.
Foreign Key Constraint On Create Table

Ø  Define multi-column FOREIGN KEY constraint when the table is created.
Multi-Column Foreign Key Constraint On Create Table

Ø  Define FOREIGN KEY constraint existing table.
Foreign Key Constraint On Existing Table

Ø  Define multi-column FOREIGN KEY constraint on existing table.
Multi-Column Foreign Key Constraint On Existing Table

Ø  Drop a FOREIGN KEY constraint

Drop A Foreign Key Constraint

Ø  Change a table schema after it’s already been defined and populated with data
Ø  Using ALTER statement followed by ADD clause
Ø  In the following example, we added d column

Changing A Schema: Add A New Column

Ø  Install the SQL Server Import Extension (click here to see installation steps)
Ø  The SQL Server Import Extension converts .txt and .csv files into a SQL table.
Ø  Once installation is complete, start Import Wizard ! Right-click on the database à click Import Wizard
Ø  Importing a file à on Import flat file wizard ! select a file by clicking Browse
Ø  Keep clicking Next to proceed à on modify columns columns can be changed
Ø  Click on Import data à click Done to finish the task
Ø  Refresh target Database and run SELECT query on the table name to retrieve the data

Import Files Into SQL Server
Imported titanic.csv file using Import Wizard

Ø  In Azure Data Studio, to save query results as CSV, Excel, and JSON, click on the file icons on the right side of the result
window.

Ø  To save a subset of the results directly à select the rows, columns, or block of interest (click and drag to select within
the result)à then Right click à A dialog will appear where to save the selection

Saving Query Results

Ø  Filtering data using WHERE clause
Ø  WHERE clause allows to retrieve specific rows instead of entire table
Ø  For e.g., we want to retrieve survived passenger list for age > 50 and sex = ‘female’ from table Titanic

Filtering Data: WHERE Clause

Ø  Filtering data using LIKE operator by constructing a wildcard (% sign)
Ø  For e.g., we want to retrieve Name that has the word ‘William’ in it from table Titanic

Filtering Data: LIKE Operator

Ø  IN operator is used to select results that match values in a list
Ø  For e.g., we want to retrieve survived passenger list where Sex column match with female from table Titanic

Filtering Data: IN Operator

Ø  Retrieve unique results from a column by removing duplicates using SELECT DISTINCT statement
Ø  To retrieve unique disease from table Disease run the following command as shown in below figure à

Removing Duplicates

Ø  Sorting results (in ascending or descending order) on single or multiple columns using ORDER BY clause
Ø  In the following example we want to sort Disease column in descending order,
Ø  and within that sort County column in ascending order, and sort Year in descending orderà

Ordering Results: ORDER BY Clause

Ø  The GROUP BY statement groups rows that have the same values into summary rows, like “find the number of cases
for each disease for the year 2018”
Ø  The GROUP BY statement is often used with aggregate functions (COUNT, MAX, MIN, SUM, AVG)
Ø  For example, we want to find the number of cases for each disease for the year 2018 from table Disease à

Grouping Data: GROUP BY Statement

Ø  The conditional expression uses CASE keyword followed by WHEN condition THEN result statement
Ø  The concept is illustrated in the following example using a test table.

Conditional Expressions

Ø  SQL performs query on related data from multiple tables using JOIN clause
Ø  Unique ID columns, automatically generated by Database system, is used to create relationships
Ø  The most common form of join is inner join (the default join)
Ø  Inner join includes rows from both tables where the join condition is met
Ø  The less common form of join is outer join: left outer join, right outer join, and full outer join
Ø  A left outer join includes the rows where the condition is met and also includes all the rows from the
table on the left, where the condition is not met
Ø  A right outer join includes the rows where the condition is met and also includes all the rows from the
table on the right, where the condition is not met
Ø  A full outer join combines the effects of the left and right joins
Ø  Many database systems do not support right and full joins

Accessing Related Tables: JOIN Clause

Ø  The concept of inner join is illustrated in the following example using item and sale tables
Ø  In the given example, JOIN clause gives the id, the quantity, and the price from sale table; and the name
and the description from the item table

JOIN Clause Example

Ø  In practice, it’s very common to have tables with many-to-many relationships
Ø  The many-to-many relationships are implemented using a junction table
Ø  The concept is illustrated in the following example using customer, item and sale tables
Ø  In the given example, JOIN clause gives the customer name and zip from customer table; the name and
the description from the item table; and the quantity and the price from sale table

Join Multiple Tables

String Functions
LEN(string): Returns the length of the given string
DATALENGTH(string): Returns string length including trailing spaces in SQL server
SUBSTRING(string, start, length) : Returns some characters from a given string
TRIM(string): Removes leading and trailing spaces from a string.
ASCII(string) Returns the ASCII value of the first character of the string
CHAR(code) Returns the character based on the ASCII code
CHARINDEX(substring, string, start) Searches for a substring in a string and returns the position
CONCAT(string1, string2, string3,…) Add two or more strings together
CONCAT_WS(separator, string1,
string2,…)
Add two or more strings together with separator
SOUNDEX(expression) Returns a four-character code to evaluate the similarity of two expressions
DIFFERENCE(expression1,
expression2)
Compares two SOUNDEX values and return an integer
REVERSE(string) Reverses a string

Ø  Length functions:
§  LEN(string) for SQL Server Or LENGTH(string) for other database system; returns the length of the given string
§  DATALENGTH(string) returns string length including trailing spaces in SQL server
§  COL_LENGTH(table_name, col_name) returns the maximum size in bytes in SQL server

Getting Length Of Strings

Ø  The function SUBSTRING(string, start, length) for SQL Server and SUBSTR(string, start, length) for other database
system; returns some characters from a given string
Getting Part Of Strings

Ø  The function TRIM(string) removes leading and trailing spaces from a string
Ø  LTRIM(string) removes leading spaces from a string
Ø  TRIM(string) removes trailing spaces from a string
Removing Spaces

Numeric Functions
ABS(number): Returns the absolute value of a number
AVG (expression): Returns the average value of an expression
COUNT(expression) : Returns the number of records returned by a select query
FLOOR(number): Returns the largest integer value that is smaller than or equal to a number
MAX(expression) Returns the maximum value in a set of values
POWER(a, b) Returns the value of a number raised to the power of another; a -base, b -exponent
RAND(seed) Returns a random number >= 0 and < 1
ROUND(number, decimals,
operation)
Rounds a number to a specified number of decimal places, number—to be rounded,
decimals—the number of decimal places to round number to, operation– (opt, default
0), if 0 it rounds the result to the number of decimal, if another value than 0, it
truncates the result to the number of decimals.
SQRT(number) Returns the square root of a number
SQUARE(number Returns the square of a number
SUM(expression) Calculates the sum of a set of values

Date And Time Functions
CURRENT_TIMESTAMP: Returns the current date and time
YEAR(date): Returns the year part for a specified date
MONTH(date): Returns the month part for a specified date
DAY(date): Returns the day part for a specified date
GETDATE(): Returns the current database system date and time (in ‘YYYY-MM-DD
hh:mm:ss:mmm’ format)
ISDATE(expression): Checks an expression and returns 1 if it is a valid date, otherwise returns 0
SYSDATETIME(): Returns the date and time of the computer where the SQL server is running
DATEADD(interval, number, date): Adds a time/date interval to a date and then returns the date
DATEDIFF(interval, date1, date2) Returns differences between two dates
DATEFROMPARTS(year, month, day) Returns a date from the specified parts (year, month, and day)
DATEPART(interval, date) Returns a specified part of a date
DATENAME(interval, date) Returns a specified part of a date

Ø  Aggregating data is the information derived from more than one row at a time
Ø  In the table Disease, we use COUNT() aggregate function to know how many cases are present in each disease
Ø  COUNT() returns a single value from a query that spans many rows of data
Ø  The GROUP BY clause groups the results before calling the aggregation function COUNT()
Ø  The HAVING clause retrieves results for a specified condition, in this case, retrieves results for COUNT(*) > 2000
Ø  And finally, execute ORDER BY Count DESC, Disease, to retrieve disease count in descending order, and Disease in
ascending order within Count.

Aggregating Data

Ø  The following statement uses the CONCAT() function to concatenate values in the First_Name and Last_Name
columns of the table Student
Concatenating Column Data

Ø  SQL query uses wildcard characters to search a pattern
Ø  The LIKE and NOT operators are used with WHERE clause to search a pattern
Ø  For example, using the wildcard “A%” we can search any string starting with a capital A
Ø  Using the wildcard “%A%” we can search all results that contain a capital A
Ø  Using the _wildcard we can search any string at specific location; for e.g., ‘_A%’ gives the results only when C is in the
second position

Searching For Patterns: Wildcard

Ø  The UNION operator combines the results of two or more SELECT statements
Ø  Each SELECT statement within UNION must have the same number of columns
Ø  The columns must also have similar data types
Ø  The columns in each SELECT statement must also be in the same order
Ø  UNION operator selects only distinct values by default. To allow duplicate values, use UNION ALL
Ø  UNION Syntax : SELECT column(s) FROM table1 UNION column(s) FROM table2
UNION Operator

Ø  A transaction is a group of operations that are handled as one unit of work If one operation fails, the entire group of
operations fail. Transaction Commands:
§  BEGIN TRANSACTION – to begin a transaction
§  COMMIT - to end and save a transaction
§  ROLLBACK – to rollback the changes

Transactions

Ø  Click on Github for example codes discussed in this presentation.
Ø  References:
1.  LinkedIn video by Bill Weinman
2.  SQL Cookbook by Anthony Molinaro
3.  SQL Notes for Professionals
4.  W3schools.com
Summary

SQL Basics

More Related Content

Similar to SQL Basics (20)

Recently uploaded (20)

SQL Basics