Instructor: Craig Duckett. Lecture 02: Thursday, March 29 th, 2018 SQL Basics and SELECT, FROM, WHERE

Transcription

1 Instructor: Craig Duckett Lecture 02: Thursday, March 29 th, 2018 SQL Basics and SELECT, FROM, WHERE 1

2 Assignment 1 is due LECTURE 5, Tuesday, April 10 th, 2018 in StudentTracker by MIDNIGHT MID-TERM EXAM is LECTURE 10, Tuesday, May 1 st, 2018 Assignment 2 is due LECTURE 12, Tuesday, May 8 th, 2018 in StudentTracker by MIDNIGHT 2

3 3 x 150 Points (450 points Total) Assignment 1 (Stage 1): DUE LECTURE 5 Tuesday, April 10 th Assignment 2 (Stage 2): DUE LECTURE 12 Tuesday, May 8 th Assignment 3 (Stage 3): DUE LECTURE 20 Tuesday, June 5 th Database Presentation: DUE LECTURE 20 Tuesday, June 5 th 3

4 Tuesday (LECTURE 1) Database Design for Mere Mortals: Chapter 1 Thursday (LECTURE 2) The Language of SQL: Chapter 1: Relational Databases and SQL Chapter 2: Basic Data Retrieval 4

5 SQL Basics SQL Syntax SQL Keywords The SELECT Statement The FROM Clause The WHERE Clause BIT275: ICE 02 5

6 SQL Basics 6

7 SQL Basics SQL, or Structured Query Language, is the common language that lies at the heart of every Relational Database Management System* (the application that actually gets the SQL to run and do something) that you'll likely to use. Now, you've probably noticed the different pronunciation already. Some people say S-Q-L, some people say sequel. I tend to say S-Q-L in general and when talking about MySQL (the MySQL official documentation says that s how it should be pronounced), but I will say Microsoft Sequel Server (because that s how they pronounce it on-campus AT Microsoft) although will sometimes revert to S-Q-L Server if I ve been talking about SQL in general just before hand. In short, both ways are correct, but I would defer to what and how they are calling it where you work Now SQL, the language, has been around since the 1970s, and that's one of the few languages I was writing 20 years ago that I'm still writing now. A lot of other programming languages have come and gone, become fashionable and unfashionable, but SQL has stuck around, and I expect it to stay for a long time to come. But SQL is a very small, very focused language, and the key to first learning it is to understand that it's a little different from other programming languages you might have come across, like C, C++, Java, or C#. SQL is what's considered a declarative query language, as opposed to procedural or imperative languages. What it means is you use SQL to describe what you want, and you let the Database Management System (DBMS) handle how that's actually done. You don't have to manually lay out the algorithm, the different steps of the procedure as you would do in other programming languages.

8 *How Does SQL Run? A.java files is simply a text file that needs a compiler like jgrasp to run. An.html or.css or.js file are also all text files that need a browser application to run. A C or C++ or C# file are simply text files as well that also need a compiler to run. Like them, a.sql file is also a text file that needs a Database Management System (DBMS)* to run, although you can also enter your SQL queries directly into the DBMS without the need of a separate.sql file (as we shall see throughout the quarter, and starting with today s ICE). Different Types of Database Management Systems Access FileMaker Pro DB2 Sybase dbase InGres PostgreSQL Rocket U2 MySQL MariaDB SQL Server Oracle See:

9 SQL Basics: How Does It Work? If I have a Books table with, perhaps, thousand different books in it, and I want to know which books have a list price of more than $40. Well, in a procedural or imperative language like C or Java, I'll have to write the steps to do this

10 SQL Basics I would probably write some (pseudo) code that would start at the first book and then would loop through all of them one by one. And every time going through this loop you're asking the question, "Is this more than $40? If so, do one thing. If not, do something else." Even writing this in pseudo code, I'd be writing a loop. I'd be writing conditions. I'd be writing returns.

11 SQL Basics With SQL, you don't describe all the steps, you just describe the outcome. You describe what you want the same way you might do in English, where you just say: or this written in SQL: That's it. Select or go get everything from the Books table where the price is more than $40. The Database Management System will take this, it will look at all your data, figure it out, and return the result set, whether that would be one book, 500 books, or even none, based on your query.

12 SQL Basics So SQL can be used to select, to retrieve, or read data, and to ask questions of it. Books Table

13 SQL Basics In fact, all of these things are referred to with the wonderful acronym of CRUD: Create, Read, Update, Delete. An SQL can be used to create not just your data, but to define the databases themselves. FYI: I will be discussing Data Manipulation Language (DML) and Data Definition Language (DDL) in Lecture 5

14 SQL Syntax 14

15 SQL Syntax 1. Comment A comment is optional text that explains you program. Comments usually describe what a program does or why code was changed. Compilers are for humans the compiler ignores them. A comment is introduced by two consecutive hypens and continues until the end of the line. -- This is a comment Some DBMS (like Microsoft SQL Server) support the /*... */ (forward slash-asterisk character pairs) multiline comments (those who've taken Java or C# should recognize these). 2. SQL Statement An SQL statement is a valid combination of tokens introduced by a keyword. Tokens are the basic indivisible parts of the SQL language; they can t be reduced grammatically. Tokens include keywords, identifiers, operators, constants, and punctuation symbols. 3. Clauses An SQL statement has one or more clauses. In general, a clause is a fragment of an SQL statement introduced by a keyword, is required or optional, and must be given in a particular order. SELECT, FROM, WHERE, and ORDER BY are examples of clauses. 4. Keywords Keywords are words that SQL reserves because they have special meaning in the language. Using a keyword outside its specific context (as an identifier, for example)) causes an error. DBMSs use a mix of standard and nonstandard (proprietary) keywords. Search your DBMS documentation for keywords or reserved words. 5. Identifiers Identifiers are words that you (or the database designer) use to name database objects such as tables, columns, aliases, indexes, and views. 6. Terminating Semicolon AN SQL statement ends with a semicolon.

16 Authors Table 16

17 SQL Syntax SQL is a "freeform" language whose statements can: Be in uppercase or lowercase (SELECT, select, or SeLECt are considered to be identical keywords) Continue on the next line as long as you don't split words, tokens, or quoted strings in two. Be on the same line as other statements Start in any column Despite its flexibility, you should adopt a consistent style. I prefer to use uppercase keywords (e.g., SELECT) and an uppercase/lowercase mix for identifiers using Pascal case (e.g., FirstName). Although I don't do it myself, some SQL coders also like to indent each clause on its own line:

18 SQL Syntax Common Errors Omitting the terminating semicolon when using multiple statements (or Transact-SQL) Misspelling a keyword or identifier Mismatched or unmatched parentheses or quotes (e.g., double quotes instead of single quotes) Listing clauses out of order Not surrounding a string or datetime literal with single quotes Surrounding a numeric constant (literal) or the keyword NULL with quotes Mismatching a table and column name Creating a table with a bad name

19 SQL Keywords 19

20 SQL Keywords Structured Query Language is the shared vocabulary of relational databases. That's not for general purpose programming. It's a small language focused purely on working with databases and the first handful of keywords that you'll learn (maybe a dozen) will take care of 90% of everything you'd ever do in SQL.

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22 The SELECT Statement See: 22

23 The SELECT Statement In terms of frequency, there's one that's head and shoulders above all the others. By far the most common word you'll ever write in SQL is SELECT. This is the word we start with when we want to SELECT or read information from one of the tables in one of our databases. We're using it to ask the database a question, and we expect a reply. Actually, the better word is not question but query. A query as in the Q in SQL: Structured Query Language. if I want to SELECT some data out of my database, I'm going to have multiple tables and multiple columns in each table. So, I need to say what specific part of this database am I interested in selecting. The general format of a SELECT statement is the word SELECT, what columns you're interested in, then the word FROM what table? SELECT and FROM are your SQL keywords. They're part of the SQL language itself. Now, the columns and table names are up to you. It's whatever you named your columns and your tables in your database. See:

24 The FROM Clause 24

25 SELECT Statement, FROM Clause

26 SELECT Statement, FROM Clause

27 SELECT Statement, FROM Clause

28 SELECT Statement, FROM Clause

29 The WHERE Clause 29

30 The WHERE Clause See:

31 The WHERE Clause See:

32 Uppercase, Lowercase, Semicolon?

33 Database Name with Dot Notation Dot notation allows you to prefix a table name to a column name to avoid ambiguity and to query multiple tables.

34 More About the WHERE Clause

35 More About the WHERE Clause

36 More About the WHERE Clause Single = sign <> NOT equal

37 More About the WHERE Clause AND OR See:

38 More About the WHERE Clause IN (note the single quotes, and comma) See:

39 More About the WHERE Clause % wildcard See: A substitute for 0 or more characters

40 More About the WHERE Clause _ wildcard A substitute for a single character See:

41 More About the WHERE Clause IS NULL See:

42 More About the WHERE Clause IS NOT NULL See:

43 BIT 275 ICE 02 43

44 BIT275: In-Class Exercises (ICE) SQL Query Exercise (solo, no partner ) Using what you've learned today about SQL, use the remainder of the class time to write SQL queries as part of the BIT275 ICE 02 exercise. Feel free to utilize whatever you might find useful in building these queries (a book, the PowerPoint slides, Google, etc).