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Retrieving Data with a SELECT Statement

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SQL for Data Science

加州大学戴维斯分校

4.6（2,578 個評分） | 110K 名學生已註冊

As data collection has increased exponentially, so has the need for people skilled at using and interacting with data; to be able to think critically, and provide insights to make better decisions and optimize their businesses. This is a data scientist, “part mathematician, part computer scientist, and part trend spotter” (SAS Institute, Inc.). According to Glassdoor, being a data scientist is the best job in America; with a median base salary of $110,000 and thousands of job openings at a time. The skills necessary to be a good data scientist include being able to retrieve and work with data, and to do that you need to be well versed in SQL, the standard language for communicating with database systems. This course is designed to give you a primer in the fundamentals of SQL and working with data so that you can begin analyzing it for data science purposes. You will begin to ask the right questions and come up with good answers to deliver valuable insights for your organization. This course starts with the basics and assumes you do not have any knowledge or skills in SQL. It will build on that foundation and gradually have you write both simple and complex queries to help you select data from tables. You'll start to work with different types of data like strings and numbers and discuss methods to filter and pare down your results. You will create new tables and be able to move data into them. You will learn common operators and how to combine the data. You will use case statements and concepts like data governance and profiling. You will discuss topics on data, and practice using real-world programming assignments. You will interpret the structure, meaning, and relationships in source data and use SQL as a professional to shape your data for targeted analysis purposes. Although we do not have any specific prerequisites or software requirements to take this course, a simple text editor is recommended for the final project. So what are you waiting for? This is your first step in landing a job in the best occupation in the US and soon the world!

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您將學習的技能

Data Science, Data Analysis, Sqlite, SQL

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4.6（2,578 個評分）

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MS

Mar 11, 2019

Great course i had a very good learning experience from this course and what i was expecting from this course i got that knowledge and i am very happy to take this course. thank you so much.

NP

Jan 07, 2018

A nice course to get introduced to writing SQL queries in Data science. Provides hands on exercises that boost confidence. Genuinely appreciate the ease with which SQL topics are covered

從本節課中

Getting Started and Selecting & Retrieving Data with SQL

In this module, you will be able to define SQL and discuss how SQL differs from other computer languages. You will be able to compare and contrast the roles of a database administrator and a data scientist, and explain the differences between one-to-one, one-to-many, and many-to-many relationships with databases. You will be able to use the SELECT statement and talk about some basic syntax rules. You will be able to add comments in your code and synthesize its importance.

Module Introduction1:00

What is SQL Anyway?7:26

Data Models, Part 1: Thinking About Your Data5:37

Data Models, Part 2: The Evolution of Data Models3:47

Data Models, Part 3: Relational vs. Transactional Models5:44

Retrieving Data with a SELECT Statement4:56

Creating Tables7:42

Creating Temporary Tables4:18

Adding Comments to SQL5:12

教學方

Sadie St. Lawrence
Data Scientist at VSP Global

腳本

In this video lesson, we're going to go through the backbone of retrieving data with the select statement. As we talked about previously, the majority of what data scientists are doing with SQL is retrieving data. To be able to do that and get you started, the first statement we are going to use is called the SELECT statement. After this lesson, you'll be able to write a basic SELECT statement, tell a database what table you want your data FROM, SELECT either all or particular columns from a table in a query, and limit the amount of data that is returned in a query. With the SELECT statement you're going to specify two pieces of information, what you want to select and where you want it from. Let's look at the concept using an example. So in this example, I'm going to select product name, that's a column from the table I want and then I'm going to say where I want to get it from. So I want to get it from Products. The output of this is then going to look like the column listed below. Which it has a column product name and then all of the list of products. We have shampoo, toothpaste, deodorant, and toothbrush. If you want to retrieve more than a single column from a table, then what you need to do is add the names of the individual columns together. But add a comma after you add the column name. So in this example, we'll still select from the Products table, but will also select the prod_name, the prod_id, and the prod_price. You can see below that I've written the same query in two different ways. I like to usually write it the second way because it helps me make sure I don't forget any commas after I wrote a column name that I am selecting. So, each statement is the same. One to me is just a little bit easier to read, so that's why I write it the second way. But both statements will produce the same results. Now let's say you have a table that has 20 columns and you want all of the columns in the table. Instead of having to write out each individual column which would take quite a while, there is a wildcard that you can use which is the asterisk. So you can put SELECT * and then FROM Products and this is going to go ahead and grab everything from the Products table, each individual column, and put it into your output. So that's the fundamentals of using SELECT. Anytime you're retrieving data you are going to have a SELECT statement. Because you're retrieving data you need to say something as hey, go get me something. This is what SELECT is for. And the FROM that accompanies it will always go hand-in-hand because if you're selecting something, you need to tell SQL, in the database, where to get it from. A lot of times, though, we may want to pull the whole table to get a view of it, to understand what data's in there. So we may do a SELECT star. But if there is something like five million records in it and we may just really want to get a sample of that. So just to view some of the data in table we may need to limit our results. To do this we can SELECT the columns we want from the table we want. Then after the FROM statement we're just going to put a statement that says limit, and you can put the number. For this I'm going to limit it to five. So I just want to see the first five records. Here though I've also listed the differences and how this syntax of this is written for different relational database management systems. I'm not going to spell it out individually for every statement and how it differs from the different systems. But this, as I've mentioned before, I want you to be aware of that there are differences in syntax. In the meantime, we're using SQLite. If you understand it's LIMIT 5 and then you switch over to a DB2 system, that's still something you can easily Google in terms of saying, hey, I'm using a DB2 system, and I want to be able to limit my results. What's the syntax to be able to do that? Just know that here, in this example, we're going to use a LIMIT 5 because we're working with SQLite. But you can see that for Oracle, you can use WHERE the number of row number is greater than or equal to a number. Then for DB2 you can use FETCH the first five rows. Okay, so that's it for this one. So this video's really gone over the backbone of retrieving data. I showed you a couple of examples of how to SELECT an individual column, multiple columns, a whole table and then also how you can limit your results. These are really the key things you need to know in order to understand writing basic queries.

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