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Set operations (SQL) Set operations in SQL is a type of operations which allow the results of multiple queries to be combined into a single result set. [1] Set operators in SQL include UNION, INTERSECT, and EXCEPT, which mathematically correspond to the concepts of union, intersection and set difference .
Join (SQL) A Venn diagram representing the full join SQL statement between tables A and B. A join clause in the Structured Query Language ( SQL) combines columns from one or more tables into a new table. The operation corresponds to a join operation in relational algebra. Informally, a join stitches two tables and puts on the same row records ...
Record linkage. Record linkage (also known as data matching, data linkage, entity resolution, and many other terms) is the task of finding records in a data set that refer to the same entity across different data sources (e.g., data files, books, websites, and databases). Record linkage is necessary when joining different data sets based on ...
Additionally, if your email isn't missing, but going straight to an unexpected folder, it may the normal outcome of using IMAP to access your mail. IMAP syncs your email regardless of where you use it, which means emails will be sorted based on the client's specifications. Check to see if your account been deactivated for inactivity
A non-primary key that can be used to identify only one row in a table. Alternate keys may be used like a primary key in a single-table select. Foreign. A key that has migrated to another entity. At the most basic definition, "a key is a unique identifier", [1] so unique key is a pleonasm.
Oracle's dialect of SQL provides a built-in function DECODE which can be used instead of the simple CASE expressions and considers two nulls equal. SELECT DECODE(i, NULL, 'Null Result', 0, 'Zero', 1, 'One') FROM t; Finally, all these constructs return a NULL if no match is found; they have a default ELSE NULL clause.
Select (SQL) The SQL SELECT statement returns a result set of rows, from one or more tables. [1] [2] A SELECT statement retrieves zero or more rows from one or more database tables or database views. In most applications, SELECT is the most commonly used data manipulation language (DML) command. As SQL is a declarative programming language ...
Relational algebra. In database theory, relational algebra is a theory that uses algebraic structures for modeling data, and defining queries on it with a well founded semantics. The theory was introduced by Edgar F. Codd . The main application of relational algebra is to provide a theoretical foundation for relational databases, particularly ...