Many of the smaller-scale database engines such as (Photoshop web design)
Many of the smaller-scale database engines such as dBase and MSAccess are only written for a single platform and operating system. In the case of dBase and MSAccess, that system is PC-based, running the Windows operating system. So, what are the basics of SQL? The Basics of SQL The basics of SQL consist of a number of parts. This section briefly introduces what simple SQL can do. As you read through this chapter, each is explained individually by example. . Query commands Querying a database is performed with a single command called the SELECT command. The SELECT command creates queries, and has various optional clauses that include performing functions such as filtering and sorting. Queries are used to retrieve data from tables in a database. There are various ways in which data can be retrieved from tables: Some database engines refer to SQL commands as SQL statements. Don t get confused. The two terms mean exactly the same thing. I prefer the term command. This is merely a personal preference on my part. The term clause is usually applied to subset parts of commands. . Basic query The most simple of queries retrieves all records from a single table. . Filtered query Afiltered query uses a WHERE clause to include or exclude specific records. . Sorted query Sorting uses the ORDER BY clause to retrieve records in a specific sorted order. . Aggregated query The GROUP BY clause allows summarizing, grouping, or aggregating of records into summarized record sets. Typically, aggregated queries contain fewer records than the query would produce, if the GROUP BY clause were not used. AHAVING clause can be used to filter in, or filter out, records in the resulting summarized aggregated record set. In other words, the HAVING clause filters the results of the GROUP BY clause, and not the records retrieved before aggregation. . Join query A join query joins tables together, returning records from multiple tables. Joins can be performed in various ways, including inner joins and outer joins. . Nested queries Anested query is also known as a subquery, which is a query contained within another query (a parent or calling query). Nesting implies that subqueries can be nested in multiple layers and thus a subquery itself can also be a calling query of another subquery. . Composite queries A composite query is a query that merges multiple query results together, most often using the UNION keyword. . Data change commands The following commands are used to change data in tables in a database. . INSERT The INSERT command is used to add new records to a table. . UPDATE The UPDATE command allows changes to one or more records in a table, at once. . DELETE The DELETE command allows deletion of one or more records from a table, at once. 126 Chapter 5
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