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Simple SELECT statement page 1 |
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SELECT statement retrieves rows from the database and has the most complex structure among other SQL statements. Almost any database user is capable of writing a simplest SELECT statement such as which retrieves all records from the table-type object PC; in so doing rows and columns of the result set have no order. To order columns of the result set they should be listed and separated by commas in the required order after the SELECT keyword:Here is the result set of this query.
The vertical projection of the РC table is obtained by listing the necessary fields only. For example, to get information about the processor speed and the amount of RAM in the computer run the following query: which returns the data:
It should be noted that a vertical sample may include duplicate rows in case where the sample does not include any potential key with the values uniquely identify each row in the table. In the PC table, the code field is a potential key, which is specified in addition as primary key. Since this field is not included in the query, there are listed some duplicate rows in the above result set (for example, rows 1 and 3). If unique rows are needed (say, we only need different combinations of processor speed and RAM amount, not specifications of all available PCs), use the DISTINCT keyword: Here's the result set:
Apart from DISTINCT, the ALL keyword, which explicitly ask for all rows, may also be applicable. However, ALL keyword is accepted by default. |
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