By Clare Churcher

Clare Churcher's Beginning SQL Queries is your advisor to learning the lingua franca of the database undefined: the SQL language. reliable wisdom of SQL is important to someone operating with databases, since it is with SQL that you simply retrieve info, manage information, and generate enterprise effects. realizing easy methods to write strong queries is the root for all paintings performed in SQL, and it's a origin that Clare lays good in her booklet.

  • Does no longer bore with syntax!
  • Helps you research the underlying thoughts inquisitive about querying a database, and from there the syntax is straightforward
  • Provides highly transparent examples and causes
  • Is academically sound whereas being useful and approachable

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Extra resources for Beginning SQL Queries: From Novice to Professional

Example text

Listing 2-10. Combining a Select and Project Operation SLastName, FirstName, Handicap (VMemberType = 'Junior' (Member)) As you can see, the algebra tells us how to get the result we want. First get the appropriate rows, and then get the required columns. The calculus doesn’t tell us how to carry out a series of steps; it just describes what the final set of rows will be like. Have a look at Listing 2-11. Listing 2-11. MemberType = 'Junior'} The left side of the expression in Listing 2-11 says we are going to retrieve the LastName, FirstName, and Handicap values from some row m.

Why are we doing this? Admittedly, it is over the top to introduce this notation for such a simple query, but as our queries become more complex and involve several tables, it is useful to have a way to express the criteria in an unambiguous way. Also, SQL is based on relational calculus. In Listing 1-12 if you replace the bar (|) with the SQL keyword FROM and the “and” with the keyword WHERE, then you essentially have the SQL query of Listing 1-6. Why Do We Need Both Algebra and Calculus? It would be reasonable to also ask, why do we need either?

I have been teaching queries to university students for several years. For some complicated queries, I often ask the class whether they find the calculus or algebra expressions more intuitive. The class is usually equally divided. Personally I find some queries just feel obvious in terms of relational algebra, whereas others feel much more simple expressed in relational calculus. Once I have the idea pinned down with one or the other, the translation into SQL (or some other query language) is usually straightforward.

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