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Retrieving Records from Multiple Tables
It does no good to put records in a database unless you retrieve them eventually and do something with them. That's the purpose of the SELECT statement—to help you get at your data. SELECT probably is used more often than any other in the SQL language, but it can also be the trickiest; the constraints you use to choose rows can be arbitrarily complex and can involve comparisons between columns in many tables.
The basic syntax of the SELECT statement looks like this:
SELECT selection_list # What columns to select
FROM table_list # Where to select rows from
WHERE primary_constraint # What conditions rows must satisfy
GROUP BY grouping_columns # How to group results
ORDER BY sorting_columns # How to sort results
HAVING secondary_constraint # Secondary conditions rows must satisfy
LIMIT count; # Limit on results
Everything in this syntax is optional except the word SELECT and the selection_list part that specifies what you want to retrieve. Some databases require the FROM clause as well. MySQL does not, which allows you to evaluate expressions without referring to any tables:
SELECT SQRT(POW(3,2)+POW(4,2));
In Chapter 1, we devoted quite a bit of attention to single-table SELECT statements, concentrating primarily on the output column list and the WHERE, GROUP BY, ORDER BY, HAVING, and LIMIT clauses. This section covers an aspect of SELECT that is often confusing—writing joins; that is, SELECT statements that retrieve records from multiple tables. We'll discuss the types of join MySQL supports, what they mean, and how to specify them. This should help you employ MySQL more effectively because, in many cases, the real problem of figuring out how to write a query is determining the proper way to join tables together.
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