A join is the instruction to the query engine on how to relate records in one table to records in another.
[MUSIC] 0:00 Previously, we've talked about database concepts. 0:04 Such as primary keys, foreign keys, 0:07 table relationships, normalization and set theory. 0:10 These concepts lead the foundation of how we write more powerful SQL queries. 0:14 We will revisit and 0:20 expand on these topics as we progress through the rest of this course. 0:21 And get more hands on with the data. 0:25 In previous courses, you've seen lots of examples of queries. 0:28 However, they've all been from one table at a time. 0:32 But, how can you combine related data for more than one table? 0:35 You can use something that is called a table join. 0:40 Using a join in a query tells the database to mesh the data from 0:43 two different tables together into one result set. 0:47 The two most common types of joins are inner join and outer join. 0:51 We're going to learn about each of these in detail. 0:56 Here we see two tables. 0:59 Customer and customer detail. 1:01 If we want to display the results from both the tables in the same view, 1:03 we use the keyword, join. 1:08 To tell SQL to put the data together, only selecting what we want. 1:10 Let's use a join to select the FirstName and LastName from the customer table. 1:14 And join it to the phone and date of birth from the customer detail table. 1:19 The result set generated from the query doesn't affect the original two tables. 1:24 All the data is there. 1:30 Separated. 1:31 Only the result set is shown based on the relationship. 1:32 Joins can be used in one to one, one to many, and many to many relationships. 1:36 In the next video, we'll look at the first type of join. 1:42 Inner join. 1:46
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