Grouping and analysis of records of your database tables are of practical interest in many applications. The solution of such problems by means of MySQL allows you to perform large volumes of routine work quickly and efficiently.
However, it Should be borne in mind: “purity" of the result will depend on many factors. Design MySQL group by is executed as written by the programmer, but the algorithm may depend on the execution time of the query and the sampling frequency.
In the preparation of request grouping is essential what is selected and how it is grouped. The source table contains a small number of fields, each of which has a different number of unique records. I_status field and w_status equivalent to the first code position, and second, its name. The field record number and time the record was created does not matter.
This set you can perform bulk operations, for example to determine the maximum and minimum value of a field start_timestamp.
Similarly, you can formulate a group query to determine all the positions and the number of people employed in them.
Value MySQL: group by &order by clause can be traced on the queries (i) and (iv) requests (ii) and (iii) are equivalent. But in any given situation should think about how to sort when you perform the grouping operation. In some cases, this can be a problem.
Count the number of grouped records is an important consideration when grouping. this operation is mainly used for these purposes, although its use is to identify the different rows in the table in some ways easier, in contrast to the distinct design.
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You Should note that MySQL group by does not always work against the MySQL order by clause. The question is not so much the priorities of the constructions in the logic of the query itself.
A Query can be formulated as an expression. In the simplest case an expression can consist in counting the number. The use of the MySQL group by count (*) as expression on which grouping is performed.
Here is the ascending sort order is the value specified in the order by clause.
MySQL does not restrict the programmer in the construction of expressions specifying fields that do not participate in the group, the sequence of grouped elements. But MySQL group by may not work the way the programmer thinks is right.
You Can group anything you want, but you should always strive to minimize the number of used fields.
MySQL group by is a very convenient operation on a duplicate data, it's easy to get the right information, but if the query is grouping multiple fields, it is advisable to consider other options for building a query.
Should take into account that the constructs group by and order by MySQL treats language as a simple "arithmetic" on records. Responsible for operations of grouping options built-in queries, the intelligent use of keys, Union and intersection tables.
Grouping in the context of the MySQL group by - it is quick getting generalized information, should not be loading the queries with this design deeper meaning. This can lead to unpredictable results and serious amounts of time.
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Alin Trodden - author of the article, editor
"Hi, I'm Alin Trodden. I write texts, read books, and look for impressions. And I'm not bad at telling you about it. I am always happy to participate in interesting projects."
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