Judgment SELECT groups. Aims to organize groups by the values of an attribute rather than the entire table. The group BY clause is usually used when we want to queue management software apply group BY aggregate functions, but can also be used to go here publicize the existence of the groups the syntax is: Far we have only used examples of tables, the usual situation in the entire database relational database queue management solutions is that we have more than one table, JOIN allows us to consult on various tables that are related through attributes (more specifically through foreign keys).

 

 

We should note that when the attributes have the same name in both tables by the table name plus a dot. As discussed below, the JOIN clause within the clause will allow us to queue management software make queries on different tables obtaining the same queue management solutions result as using the WHERE clause, as recommended by the American National Standards Institute (ANSI). In most engines of this database is more efficient manner.

 

Using the INNER JOIN clause allows us to join two or more tables based on the common attributes and we will return only records that have equal values in the attributes of each of the tables (it is based on the intersection the result will be those with common values in the two tables). In our example, the SELECT statement is: We can see that we have 16 records in the Books table and the last two have no name of queue management software publisher, and that the publishers have no table with the code value5 editorial.

 

 

If on the other hand we would like to useful source get all the records Editorial board whether they have records in the Books table, we use a RIGHT JOIN (it is the other option that allow outer joins). The SELECT statement should be: The UNION operator combines records with the same attributes: The result of two tables or two consultations on a new set of records. The only restriction is that the result of the union must be compatible, the number of attributes in the result should be the same. UNION operator is useful when we want to merge records that are stored in different queue management solutions tables.

 

Both Tables as SELECT statements must have the same number of attributes and attributes queue management solutions must be the same type of data, can not join two tables where one of the attributes have different data types, such numbers over and text on the other, unless we use a function to convert numbers to queue management software text. In some cases it is necessary to use the result of a query to select other values It is what we call nested queries: there is a nested sub queue management solutions query within a query. The sub query is enclosed in parentheses. The sub query can return a single result or multiple results.