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
… 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
…
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
… 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
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
… 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 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
… 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
… 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
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