By Clare Churcher
Starting Queries with SQL is a pleasant and simply learn advisor to writing queries with the all-important ― within the database international ― SQL language. an individual who does any paintings in any respect with databases must recognize whatever of SQL, and that's evidenced by means of 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 means of the writer of starting Database layout, an writer who's garnering nice reports on Amazon as a result readability and succinctness of her writing
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Additional info for Beginning SQL Queries
If we do this only those rows with a value in the Handicap field will be included in the count. SELECT COUNT(Handicap) FROM Member We can also specify that we want to count the number of unique values for an attribute. If we want to know how many different values of MemberType appear in the Member table then we can use the following query: SELECT COUNT(DISTINCT MemberType) FROM Member It is worth reiterating that different database software will support different parts of the SQL standard syntax.
For a join we have the extra condition that we want to retrieve only those combinations of rows where the membership type from each table is the same. Type. The pair of rows depicted in Figure 3-5 satisfies that condition and so will be retrieved. If m stays where it is and t moves down a row, then the condition will no longer be satisfied and the new combination will not be included. 37 CHAPTER 3 ■ A FIRST LOOK AT JOINS Figure 3-5. Type; If we look carefully at the preceding statement we can see that the first two lines represent the Cartesian product, and the WHERE clause in last line is selecting a subset of the rows where the membership types are the same.
This ensures that every row is unique. 3 Let’s look at an example. Consider retrieving just the FirstName column from the Member table. Figure 2-8 shows two possible results. 3 Formally, in terms of relational algebra, the result of every operation will generate another relation or set of unique rows. See Appendix 2 for more information. 25 CHAPTER 2 ■ SIMPLE QUERIES ON ONE TABLE Figure 2-8. Projecting the FirstName column from the Member table It is useful to think about why we might carry out a query retrieving just names.
Beginning SQL Queries by Clare Churcher