Delete duplicate MySQL distinct

If semantic organization of the information embodied in reality, the scope of design MySQL distinct keyword would have instantly self-destructed. Modern database built in the framework of relational data relationships, so the task of deleting duplicate records are relevant.

The Appearance of identical strings, as a rule, is not a problem that cannot be solved, but to avoid duplication of the contents of table fields is practically impossible in many cases.

Organization of the database

It is believed that “correct” the database has a unique table, and each of them contains a unique field. Allows the presence of identical content in the fields of different tables only when they are key and it is the logical link data.

For Example, a diagram of the staffing will come out of table of data about employees for a specific field. The staffing table only contains what it refers to in the context of a particular company and a list of employees only contains personal data of staff.

With this option MySQL distinct will work on the query to both tables that links staffing with employees.

The uniqueness of the tables and fields

In the interaction table the staffing table and the list of employees for each row of the first table has a special place in the second. The second table can contain the same surnames, names, patronymics of people, addresses of living in the city can also contain identical street. Numbers of houses and apartments may not matter that much (they don't take up much space).

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Ideally, all of the same words placed in different tables and they correspond to a unique key. For example, a list of all the streets, names, names, patronymics. In the employee table the primary schema merge in the correct version of the submission, and to table the staffing table is connected not with a list of employees and the query for it, and those that are associated with it.

The more classified the information, the more important to use the MySQL distinct keyword. For the proper organization of data has “pay” - when joining tables total number of rows from the sample increases in proportion to the number of rows in each table. This is an abstract example, usually the developer does not detail information to such an extent. Use MySQL distinct solves this problem: it will only select the desired entry.

Can stand the task of parsing the paragraphs into sentences, sentences into phrases, and phrases into words. In this case, without a dictionary is not enough, and it will have to make a dictionary conjugations, endings and other elements of the language syntax.

Example of fetching a MySQL query "select distinct ..."

The table has records in which the four seasons and two state of entry: active and passive. Examples fetch:

• All records;
• Only unique
• Unique within the conditions.

They can be like indicated on the image in the article.

The functionality of the operator fetching only unique records will satisfy any data structures. You can use a query in the query, group and sort data before selecting.

However, it is always preferable to simplify the work with the database. Using MySQL distinct on a single field is always preferable than working on several.

It is Particularly important to formulate queries that combine multiple tables. Any entity data in the relational databases, before you start to function in the where clause and the join leads to large amounts of data. Orientation in them requires care and patience from the developer.