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  2. Join (SQL) - Wikipedia

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

    An inner join (or join) requires each row in the two joined tables to have matching column values, and is a commonly used join operation in applications but should not be assumed to be the best choice in all situations. Inner join creates a new result table by combining column values of two tables (A and B) based upon the join-predicate.

  3. Set operations (SQL) - Wikipedia

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

    In SQL the UNION clause combines the results of two SQL queries into a single table of all matching rows. The two queries must result in the same number of columns and compatible data types in order to unite. Any duplicate records are automatically removed unless UNION ALL is used.

  4. Merge (SQL) - Wikipedia

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

    Additionally there is a single-row version, UPDATE OR INSERT INTO tablename (columns) VALUES (values) [MATCHING (columns)], but the latter does not give you the option to take different actions on insert versus update (e.g. setting a new sequence value only for new rows, not for existing ones.)

  5. Relational algebra - Wikipedia

    en.wikipedia.org/wiki/Relational_algebra

    The relational algebra uses set union, set difference, and Cartesian product from set theory, and adds additional constraints to these operators to create new ones.. For set union and set difference, the two relations involved must be union-compatible—that is, the two relations must have the same set of attributes.

  6. Schema matching - Wikipedia

    en.wikipedia.org/wiki/Schema_matching

    The terms schema matching and mapping are often used interchangeably for a database process. For this article, we differentiate the two as follows: schema matching is the process of identifying that two objects are semantically related (scope of this article) while mapping refers to the transformations between the objects.

  7. Where (SQL) - Wikipedia

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

    If multiple combinations are applied, parentheses can be used to group combinations to indicate the order of evaluation. Without parentheses, the AND operator has a stronger binding than OR . The following example deletes rows from mytable where the value of mycol is greater than 100, and the value of item is equal to the string literal 'Hammer':

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  9. Select (SQL) - Wikipedia

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

    Title Authors ----- ----- SQL Examples and Guide 4 The Joy of SQL 1 An Introduction to SQL 2 Pitfalls of SQL 1 Under the precondition that isbn is the only common column name of the two tables and that a column named title only exists in the Book table, one could re-write the query above in the following form: