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. a person who does any paintings in any respect with databases must comprehend whatever 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 by way of the writer of starting Database layout, an writer who's garnering nice stories on Amazon as a result of readability and succinctness of her writing
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24 CHAPTER 2 ■ SIMPLE QUERIES ON ONE TABLE Expression 1 Expression Expression 1 F ? T T F ? F F F F ? F ? Expression 2 Expression 2 T T F ? T F ? T T T T F T ? F T F ? T ? a) AND b) OR c) NOT Figure 2-7. Truth tables with three-valued logic (T = True, F = False, ? = Don’t know) Managing Duplicates If our tables have been designed well, they will have a primary key. This ensures that every row is unique. 3 Let’s look at an example. Consider retrieving just the FirstName column from the Member table.
We do not need these abstract ideas for simple queries. However, if all queries were simple, you would not be reading this book. In the first instance, queries are expressed in everyday language that is often ambiguous. ” This can mean different things depending on where you insert commas. For example a comma after “20” leads to the interpretation that everyone under 20 is included, while a comma after “home” suggests that they must also get an allowance. Even after we have sorted out what the naturallanguage expression means, we then have to think about the query in terms of the actual tables in the database.
Different database products may interpret join compatibility differently. Some might let you join on a float (number with a decimal point) in one table and an integer in another. Some may be fussy about whether text fields are the same length (for example CHAR(10) or CHAR(15)), and others may not. I recommend you don’t try to join on fields with different types unless you are very clear what your particular product does. The best strategy, as always, is to think carefully when you design your tables.