By Clare Churcher
Starting Queries with SQL is a pleasant and simply learn advisor to writing queries with the all-important ― within the database global ― SQL language. somebody who does any paintings in any respect with databases must comprehend anything of SQL, and that's evidenced through the robust 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 because of the readability and succinctness of her writing
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Example text
Some products do this extremely well, others not so well. Many products have analyzer tools that will let you see in what order things are being done. For many queries, writing your SQL differently doesn’t make much difference, but you can make things more efficient by providing indexes for your tables. We will look at these issues more closely in Chapter 9. 41 CHAPTER 3 ■ A FIRST LOOK AT JOINS An Outcome Approach The reason that the way we write our SQL statements often doesn’t affect the efficiency of a query is that SQL is fundamentally based on relational calculus, which describes the criteria the retrieved rows must meet.
There are often several equivalent ways of expressing an SQL statement. Some ways are very much based on calculus, some are based on algebra, and some are a bit of both. During my time as a university lecturer I often asked the class whether they found the calculus or algebra expressions more intuitive for a particular query. The class was usually equally divided. Personally, I find that some queries just feel obvious in terms of relational algebra, whereas others feel much more simple when expressed in relational calculus.
We just need to select the rows satisfying the conditions about the year and tournament name by adding a WHERE clause, and then project the name attributes by specifying them in the SELECT clause. TourID WHERE TourName = 'Leeston' AND Year = 2014; Order of Operations In the description in the previous section, we joined all the tables first and then selected the appropriate rows and columns. The result of the join is an intermediate table (as in Figure 3-7) that is potentially extremely large if there are lots of members and tournaments.