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  2. Referential integrity - Wikipedia

    en.wikipedia.org/wiki/Referential_integrity

    For referential integrity to hold in a relational database, any column in a base table that is declared a foreign key can only contain either null values or values from a parent table's primary key or a candidate key. [2] In other words, when a foreign key value is used it must reference a valid, existing primary key in the parent table.

  3. List of SQL reserved words - Wikipedia

    en.wikipedia.org/wiki/List_of_SQL_reserved_words

    Reserved words in SQL and related products In SQL:2023 [3] In IBM Db2 13 [4] In Mimer SQL 11.0 [5] In MySQL 8.0 [6] In Oracle Database 23c [7] In PostgreSQL 16 [1] In Microsoft SQL Server 2022 [2]

  4. Data validation - Wikipedia

    en.wikipedia.org/wiki/Data_validation

    Checks that data is present, e.g., customers may be required to have an email address. Range check Checks that the data is within a specified range of values, e.g., a probability must be between 0 and 1. Referential integrity Values in two relational database tables can be linked through foreign key and primary key.

  5. SQL syntax - Wikipedia

    en.wikipedia.org/wiki/SQL_syntax

    A derived table is the use of referencing an SQL subquery in a FROM clause. Essentially, the derived table is a subquery that can be selected from or joined to. The derived table functionality allows the user to reference the subquery as a table. The derived table is sometimes referred to as an inline view or a subselect.

  6. Select (SQL) - Wikipedia

    en.wikipedia.org/wiki/Select_(SQL)

    Although not in standard, most DBMS allows using a select clause without a table by pretending that an imaginary table with one row is used. This is mainly used to perform calculations where a table is not needed. The SELECT clause specifies a list of properties (columns) by name, or the wildcard character (“*”) to mean “all properties”.

  7. Why did I receive an email from MAILER-DAEMON? - AOL Help

    help.aol.com/articles/what-is-a-mailer-daemon...

    The most common reason for a failed delivery is that the email address entered isn't valid. If the delivery failure message says the account doesn't exist double check the spelling of the address you entered. A single misplaced letter could cause a delivery failure.

  8. Unique key - Wikipedia

    en.wikipedia.org/wiki/Unique_key

    Here ID serves as the primary key in the table 'Author', but also as AuthorID serves as a Foreign Key in the table 'Book'. The Foreign Key serves as the link, and therefore the connection, between the two related tables in this sample database. In a relational database, a candidate key uniquely identifies each row of data values in a database ...

  9. Data definition language - Wikipedia

    en.wikipedia.org/wiki/Data_definition_language

    CREATE TABLE employees (id INTEGER PRIMARY KEY, first_name VARCHAR (50) not null, last_name VARCHAR (75) not null, mid_name VARCHAR (50) not null, dateofbirth DATE not null); Some forms of CREATE TABLE DDL may incorporate DML ( data manipulation language )-like constructs, such as the CREATE TABLE AS SELECT (CTaS) syntax of SQL.