CREATE VIEW

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CREATE VIEW

CREATE VIEW — Creates a view into a table, optimizing access to a summary of its contents.

Synopsis

CREATE VIEW view-name ( view-column-name [,...] )
AS SELECT { column-name | selection-expression } [AS alias] [,...]
FROM table-name
[WHERE [NOT] boolean-expression [ {AND | OR} [NOT] boolean-expression]...]
[GROUP BY { column-name | selection-expression } [,...]]

Description

The CREATE VIEW statement creates a view of a table with selected columns and aggregates. VoltDB implements views as materialized views. In other words, the view is stored as a special table in the database and is updated each time the corresponding database table is modified. This means there is a small, incremental performance impact for any inserts or updates to the table, but selects on the view will execute efficiently.

The following limitations are important to note when using the CREATE VIEW statement with VoltDB:

  • Views are allowed on individual tables only. Joins are not supported.

  • The SELECT statement must include a field specified as COUNT(*). Other aggregate functions (COUNT, MAX, MIN, and SUM) are allowed following the COUNT(*).

  • If the SELECT statement contains a GROUP BY clause, all of the columns and expressions listed in the GROUP BY must be listed in the same order at the start of the SELECT statement.

Examples

The following example defines a view that counts the number of records for a specific product item grouped by its location (that is, the warehouse the item is in).

CREATE VIEW inventory_count_by_warehouse (
     productID,
     warehouse,
     total_inventory
) AS SELECT
     productID,
     warehouse,
     COUNT(*)
FROM inventory GROUP BY productID, warehouse;

The next example uses a WHERE clause but no GROUP BY to provide a count and minimum and maximum aggregates of all records that meet a certain criteria.

CREATE VIEW small_towns ( number, minimum, maximum )
    AS SELECT count(*), min(population), max(population) 
    FROM TOWNS WHERE population < 10000;
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