The Data Definition Language is the part of the SQL language that allows you to perform operations on the objects that make up the structure of a database (i.e. the container). These objects are TABLES, VIEWS or INDEX. (The present section focuses on the manipulation of tables and indexes, the…
The UNION clause makes it possible to add the result of two or more queries. Exactly as if we were doing only one query from several separate queries. select statement 1 UNION select statement 2 UNION select statement 3 … In addition, the UNION clause naturally eliminates the resulting duplicates.…
A subquery is a SELECT statement that can be used against an unknown value (which is then known when the subquery is performed). In fact, subqueries are an indirect way to relate multiple tables, since the subquery can be built on a table other than the main query. A subquery…
1.1 Statistical functions The statistical functions are standard to SQL and allow the calculation of statistical results on the rows of a table. AVG (column) Average values SUM (column) Sum of values MIN (column) Minimum value MAX (column) Maximum value COUNT (column) Number of values in the column COUNT (DISTINCT…
When two tables are matched in a query, a join is made. The goal is to relate the data of one table with the data of another table. The result is to have the rows of two tables side by side. For this to be feasible and for the relationship…
1.1 The conditional expression SQL determines on which lines will be performed an operation by means of a conditional expression in the WHERE clause. At each line of the table, the expression conditional is re-evaluated based on line-specific values. If the expression evaluates to TRUE, then the requested operation (SELECT,…
1.1 A simple request The SELECT statement is used to query one or more rows of one or more tables. Whereas the other instructions, seen so far, have only a very small number of clauses, the SELECT statement is the one that understands the most and is the very fact,…
The Data Manipulation Language is the part of the SQL language used to perform operations on the content (LINES) of one or more tables. There are only four possible operations: the insertion of rows, the withdrawal of rows, the interrogation of rows and changing the values of one or more…
1.1 Structure of a relational database TABLE Any information in a relational database is in the form of a table (like a spreadsheet). This one is a collection of columns (its structure) and lines (its content). Each of the tables represents a type of object or specific concept. For example,…