Database Query 1: SQL Basics

J.G. Zheng

Fall 2010

CIS 3730Designing and Managing Data

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Overview

Using Structured Query Language (SQL) to get the data you want from relational databases

Learning basic syntax and simple queries

All examples can be used with the “Northwind” database

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SQL: Introduction

SQL is a standard command language use by relational DBMS to perform database operations

Some facts about SQL English-like Case insensitive An ANSI standard Not considered to be a programming language Most venders have their own proprietary extensions,

making SQL slightly different in different DBMS products

What Does SQL Do?

DML - data manipulation language Retrieving data: database queries Manipulating data: insert, update, delete

DDL - data definition language Defining and modifying data structures

(schema): databases, tables, views, etc.

DCL - data control language Control data access: permissions, etc.

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Database Queries

A query is a request for information from a database.

“SELECT” statements are used to retrieve data The result of a query is generally a table (but not

necessarily a relation)

SQL SELECT basics Defining selection criteria

Sorting

Table join

SELECT: General Syntax

SELECT [Column(s), or other expressions]

FROM [Table(s)]

[WHERE …]

[ORDER BY …]

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1.1 Select Columns

Syntax SELECT * (or a list of columns)

FROM TableName

Example

SELECT * FROM Products;

SELECT ProductName, UnitPriceFROM Products;

“*” represents all columns in the table

Provide a list of columns separated by comma

Column Qualifier and Alias

Qualifiers are used to distinguish column names when there is ambiguity

SELECT Products.ProductName, Products.UnitPriceFROM Products

Alias is used to give column another name

SELECT UnitPrice*Quantity AS Total

FROM [Order Details];Use “AS” operator to give an alias.

A qualifier of table name

Use square brackets if there is blank spaces in table or column names.

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2. Select Rows (Selection Criteria)

Use WHERE clause to specify selection criteria

Common comparison operators =, >, <, >=, <=, <>, !=

Examples

SELECT * FROM Products WHERE UnitPrice = 18;

SELECT BookTitle, ListPrice FROM ProductsWHERE UnitPrice < 20;

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2.1 Comparing with ConstantsNumbers (int, float, decimal, etc.) … WHERE UnitPrice > 20.99

Text (char, varchar, nvarchar, etc.): use '…' (single quote) for values … WHERE ProductName = 'Tofu' // exact match … WHERE ProductName > 'Chang'

Data/Time: SQL Server uses '...' … WHERE OrderDate < '08/30/2008'

Boolean (bit, yes/no, etc.): 'TURE' or 1, 'FALSE' or 0 … WHERE Discontinued = 1 … WHERE Discontinued = 'true'

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compared by alphabetical order, “>” means after

Before the date, 12:00AM

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2.2 IN and BETWEEN

IN (a value list): equals to any value in the list

SELECT * FROM ProductsWHERE UnitPrice IN (18, 19, 21)

BETWEEN min AND max: any value falls in the range, inclusive

SELECT * FROM ProductsWHERE UnitPrice BETWEEN 10 AND 20

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2.3 String Pattern Match: LIKE

LIKE: keyword match, or fuzzy query _ (underscore): matching one single character

% (percentage) wildcard: matching any multiple characters

Comparison pattern is case insensitive

Example Can I get all tofu related products?

SELECT * FROM Products

WHERE ProductName LIKE '%tofu%'

Matching any characters after “tofu”

Matching any characters before “tofu”

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2.4 IS NULL

NULL means missing value

IS NULL, IS NOT NULL

SELECT * FROM Orders

WHERE ShipPostalCode IS NULL

SELECT * FROM Orders

WHERE ShipPostalCode IS NOT NULL

2.5 NOTReversal criteria NOT (expression)

Examples

… WHERE NOT UnitPrice > 20;

… WHERE NOT ProductName LIKE '%tofu%';

… WHERE ProductName NOT LIKE '%tofu%';

… WHERE NOT ShipPostalCode IS NULL;

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2.6 Compound ConditionsUse logical operators to connect multiple criteria AND: satisfy both conditions OR: satisfy either criterion Comparison precedence: AND has a higher precedence over OR Best practice: use parentheses () to explicitly define comparison

order

Examples

… WHERE UnitPrice <= 20 AND UnitPrice >= 10;

… WHERE ShippedDate ='8/2/1996' OR ShippedDate = '8/9/1996';

… WHERE (City = 'London' OR City = 'Seattle') AND HireDate > '1/1/1993';

If () are not used, “ City = 'Seattle' AND HireDate > '1/1/1993' ” will be evaluated first, which is wrong.

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3. SortingSorting query results ORDER BY Column(s) [ASC/DESC] ASC (ascending) is the default order

Examples

SELECT * FROM Products ORDER BY UnitPrice;SELECT * FROM Products ORDER BY 6;

Multiple sorting

SELECT * FROM ProductsORDER BY CategoryID, UnitPrice DESC

Column position can also be used (starting from 1)

Sort by category first, then UnitPrice

4. Table Join

Retrieving data from multiple tables The query result consists of columns from

more than one table

How do rows match from different tables? Cross Join: no need to match.

Inner Join: use the foreign key constraint as the matching criteria

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Table Join Effect

Product Category

A 1

B 2

Category Name

1 Canned

2 Drink

3 Fresh

Product Category

A Canned

B Drink

Product Category

A Canned

A Drink

A Fresh

B Canned

B Drink

B Fresh

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Cross Join: no row matching

Inner Join: row matching based on foreign key

FK

Join

Table Join SQL Example

What is the category name for each product?

SELECT ProductName, CategoryName

FROM Products, Categories

Where

Products.CategoryID = Categories.CategoryID

Joining/matching criteria: very important, don’t forget!

Use “table.column” format to avoid ambiguity – CategoryID in which table?

Primary keyForeign key

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3. Table Join Syntax Alternative

SELECT ProductName, CategoryNameFROM Products, Categories

Where Products.CategoryID = Categories.CategoryIDAND Discontinued = 0;

VS.

SELECT ProductName, CategoryNameFROM Products INNER JOIN Categories ON

Products.CategoryID = Categories.CategoryIDWHERE Discontinued = 0;

1. Joining/matching criteria: very important, don’t forget!

2. Table.Cloumn format is used to avoid ambiguity.

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Joining More Tables (1)

For each order after April 1, 1998, I want to see what products (product name) were ordered, and how many.

SELECT Orders.OrderID, OrderDate, ProductName, QuantityFROM Orders, [Order Details], ProductsWHERE

Orders.OrderID = [Order Details].OrderIDAND [Order Details].ProductID = Products.ProductIDAND OrderDate > '1998-04-01'

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Joining More Tables (2)

For each order after April 1, 1998, I want to see what products (product name) were ordered, and how many.

SELECT Orders.OrderID, OrderDate, ProductName, Quantity

FROM

Orders INNER JOIN [Order Details] ON

Orders.OrderID = [Order Details].OrderID

INNER JOIN Products ON

[Order Details].ProductID = Products.ProductID

WHERE OrderDate > '1998-04-01'

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SummaryKey concepts Database query SQL: DML, DDL, DCL Join, cross join, inner join

Key skills: write SQL SELECT statement to retrieve data; and know the result of a given SQL SELECT statement. Select columns, * Selection criteria in WHERE: comparison operators

=, <, >, >=, <=, <>, !=, IN, BETWEEN, LIKE, IS NULL, NOT %, _ AND, OR

ORDER BY Table join

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More SQL ResourcesW3Schools SQL Tutorial http://www.w3schools.com/sql/

SQL Course http://sqlcourse.com/ http://sqlcourse2.com/

A gentle introduction to SQL http://sqlzoo.net/

Other http://www.youtube.com/watch?v=Rpp28U_K9Lk http://www.1keydata.com/sql

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