Together, these four statements make up the primary data manipulation language (DML) commands of SQL. Three other common statements, INSERT, UPDATE, and DELETE, let you manipulate the records stored in each table. The primary statement, SELECT, provides basic data selection and retrieval features. The most popular of these languages, SQL, uses simple English-like sentences to join, order, summarize, and retrieve just the data values you need. To join tables together, relational databases implement query languages that allow you to manipulate the data using relational algebra (from which the term relational database derives). In each table, Iâve put an asterisk next to the column title that acts as the primary key column.įigure 4-1. Three tables, and yet they work as one For the sample orders in Table 4-1, the data could be normalized into three separate tables: one for order line items, one for customers, and one for products (see Table 4-2, Table 4-3, and Table 4-4, respectively). By breaking the data into separate tables with data subsets, assigning a unique identifier to each record/row in every table (a primary key), and making a few other adjustments, the data could be ânormalizedâ for both processing efficiency and data integrity. But instead of just sitting around and complaining about them like I do, he came up with a solution: normalization. Codd, the brilliant computer scientist that he was, saw these problems, too. Also, although the product ID âBEV01COFâ indicates coffee, one of the lines lists it as âTea.â A few other problems are inherent in data thatâs placed in a single flat file database table. Customer names and product names repeat multiple times. Unfortunately, this table of orders has a lot of repetition. The important data appears at a glance in a nice and orderly arrangement, and itâs easy to sort the results based on a particular column. Putting all of your information in a table is really convenient. Table 4-1 presents a table of orders, with a separate record for each line item of the order. For convenience, tables are presented as a grid of data values, with each row representing a single record and each column representing a consistent field that appears in each record. The relational databases that most programmers use collect data in tables, each of which stores a specific set of unordered records. Fortunately, you donât need to know anything about these terms to use relational database systems. Date, another âreal programmer.â Upon reading that 1970 paperâand if you have a free afternoon, you would really benefit from spending time with your family or friends rather than reading that paperâyou will enter a world of n-tuples, domains, and expressible sets. In 1970, he issued âA Relational Model of Data for Large Shared Data Banks,â the seminal paper on relational modeling, and later expanded on the basic concepts with C. Such databases are built on the ârelational modelâ designed by Edgar Codd of IBM. These days, when we think of âdatabase,â itâs generally a relational database system.
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