By Clare Churcher
Starting Queries with SQL is a pleasant and simply learn consultant to writing queries with the all-important ― within the database global ― SQL language. a person who does any paintings in any respect with databases must comprehend anything of SQL, and that's evidenced through the powerful revenues of such books as studying SQL (O'Reilly) and SQL Queries for Mere Mortals (Pearson). starting Queries with SQL is written through the writer of starting Database layout, an writer who's garnering nice experiences on Amazon end result of the readability and succinctness of her writing
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Extra resources for Beginning SQL Queries
Cartesian product followed by selecting a subset of rows The operation shown in Figure 3-3 (a Cartesian product followed by selecting a subset of rows) is known as an inner join (often just called a join). The condition we use to select the rows is known as the join condition. Type; The keyword INNER JOIN is used, and we can see the condition for selecting the rows after the keyword ON. Once again, you may find that some versions of SQL do not support the phrase INNER JOIN; however, we will see other ways to express the query later in this chapter.
Process Approach One way to approach a query is to think in terms of the operations we need to carry out on the tables. Let’s think about how we might to get a list of names for members who practice on a Monday. We might imagine first retrieving just the rows from the Team table that have Monday in the PracticeNight column. We might then join those rows with the Member table (more about joins later) and then extract the names from the result. We will call this the process approach, as it is a series of steps carried out in a particular order.
This is another type of question that can’t be answered with a simple WHERE clause that looks at independent rows in a table. In fact, we can’t even answer this question with a query that involves only the Entry table. Member 138, Michael Stone, has not entered tournament 38, but he doesn’t even get a mention in the Entry table because he has never entered any tournaments at all. We’ll see how to deal with questions like this in Chapter 7. Summary In this chapter, we have looked at queries on a single table.