Database Query 1: SQL Basics

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1 Database Query 1: SQL Basics CIS 3730 Designing and Managing Data J.G. Zheng Fall

2 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 2

3 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 3

4 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. 4

5 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 5

6 SELECT: General Syntax SELECT [Column(s), or other expressions] FROM [Table(s)] [WHERE ] [ORDER BY ] 6

7 1.1 Select Columns Syntax SELECT * (or a list of columns) FROM TableName Example SELECT * FROM Products; * represents all columns in the table SELECT ProductName, UnitPrice FROM Products; Provide a list of columns separated by comma 7

8 Column Qualifier and Alias Qualifiers are used to distinguish column names when there is ambiguity SELECT Products.ProductName, Products.UnitPrice FROM Products A qualifier of table name Alias is used to give column another name SELECT UnitPrice*Quantity AS Total FROM [Order Details]; Use AS operator to Use square brackets if there is blank spaces in table or column names. give an alias. 8

9 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 Products WHERE UnitPrice < 20; 9

10 2.1 Comparing with Constants Numbers (int, float, decimal, etc.) WHERE UnitPrice > 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' compared by alphabetical order, > means after Before the date, 12:00AM Boolean (bit, yes/no, etc.): 'TURE' or 1, 'FALSE' or 0 WHERE Discontinued = 1 WHERE Discontinued = 'true' 10

11 2.2 IN and BETWEEN IN (a value list): equals to any value in the list SELECT * FROM Products WHERE UnitPrice IN (18, 19, 21) BETWEEN min AND max: any value falls in the range, inclusive SELECT * FROM Products WHERE UnitPrice BETWEEN 10 AND 20 11

12 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 before tofu Matching any characters after tofu 12

13 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 13

14 2.5 NOT Reversal criteria NOT (expression) Examples WHERE NOT UnitPrice > 20; WHERE NOT ProductName LIKE '%tofu%'; WHERE ProductName NOT LIKE '%tofu%'; WHERE NOT ShipPostalCode IS NULL; 14

15 2.6 Compound Conditions Use 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. 15

16 3. Sorting Sorting 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 Products ORDER BY CategoryID, UnitPrice DESC Column position can also be used (starting from 1) Sort by category first, then UnitPrice 16

17 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 17

18 Table Join Effect Product Category A 1 B 2 FK Join Category Name 1 Canned 2 Drink 3 Fresh Product A A A B B B Category Canned Drink Fresh Canned Drink Fresh Product A B Cross Join: no row matching Category Canned Drink Inner Join: row matching based on foreign key 18

19 Table Join SQL Example What is the category name for each product? SELECT ProductName, CategoryName FROM Products, Categories Where Foreign key Products.CategoryID = Categories.CategoryID Primary key Use table.column format to avoid ambiguity CategoryID in which table? Joining/matching criteria: very important, don t forget! 19

20 3. Table Join Syntax Alternative SELECT ProductName, CategoryName FROM Products, Categories Where Products.CategoryID = Categories.CategoryID AND Discontinued = 0; VS. 1. Joining/matching criteria: very important, don t forget! 2. Table.Cloumn format is used to avoid ambiguity. SELECT ProductName, CategoryName FROM Products INNER JOIN Categories ON Products.CategoryID = Categories.CategoryID WHERE Discontinued = 0; 20

21 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, Quantity FROM Orders, [Order Details], Products WHERE Orders.OrderID = [Order Details].OrderID AND [Order Details].ProductID = Products.ProductID AND OrderDate > ' ' 21

22 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 > ' ' 22

23 Summary Key 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 23

24 More SQL Resources W3Schools SQL Tutorial SQL Course A gentle introduction to SQL Other