Class Union

Namespace

YndigoBlue.Velocity.Model

Assembly

YndigoBlue.Velocity.dll

Represents a UNION operation that combines the result sets of two or more queries.

public class Union

Inheritance

object

Union

Remarks

UNION combines rows from multiple Query objects into a single result set. All queries in a union must return the same number of columns with compatible data types.

There are two types of UNION operations:

• UNIONRemoves duplicate rows from the combined result set
• UNION ALLIncludes all rows, even duplicates (faster performance)

Unions are added to a base query using Union(Union). Multiple unions can be chained together to combine many queries.

Examples

Simple UNION of two queries:

Query activeQuery   = new Query().Select(activeUsersTable["username"]).From(activeUsersTable);
Query archivedQuery = new Query().Select(archivedUsersTable["username"]).From(archivedUsersTable);

// Combine with UNION (removes duplicates)
activeQuery.Union(new Union(archivedQuery, unionAll: false));

using (ResultSet results = manager.Retrieve(activeQuery)) { ... }

// SQL: SELECT username FROM active_users
//      UNION
//      SELECT username FROM archived_users

UNION ALL to keep duplicates:

// Get all order totals from both current and historical orders
Query currentOrders    = new Query().Select(ordersTable["total"]).From(ordersTable);
Query historicalOrders = new Query().Select(orderHistoryTable["total"]).From(orderHistoryTable);

// Combine with UNION ALL (keeps duplicates, faster)
currentOrders.Union(new Union(historicalOrders, unionAll: true));

using (ResultSet results = manager.Retrieve(currentOrders)) { ... }

// SQL: SELECT total FROM orders
//      UNION ALL
//      SELECT total FROM order_history

Multiple unions:

// Combine data from three different tables
Query emailsQuery    = new Query().Select(emailsTable["email_address"]).From(emailsTable);
Query customersQuery = new Query().Select(customersTable["email"]).From(customersTable);
Query vendorsQuery   = new Query().Select(vendorsTable["contact_email"]).From(vendorsTable);

// Chain multiple unions
emailsQuery.Union(new Union(customersQuery, false));
emailsQuery.Union(new Union(vendorsQuery, false));

// SQL: SELECT email_address FROM emails
//      UNION
//      SELECT email FROM customers
//      UNION
//      SELECT contact_email FROM vendors

UNION with WHERE clauses:

Query activeUsers = new Query()
    .Select([ usersTable["username"], usersTable["email"] ])
    .From(usersTable)
    .Where(new Criterion<string>(usersTable["status"], "active"));

Query activeAdmins = new Query()
    .Select([ adminsTable["username"], adminsTable["email"] ])
    .From(adminsTable)
    .Where(new Criterion<string>(adminsTable["status"], "active"));

activeUsers.Union(new Union(activeAdmins, false));

// SQL: SELECT username, email FROM users WHERE status = 'active'
//      UNION
//      SELECT username, email FROM admins WHERE status = 'active'

Constructors

Union(Query, bool)

Creates a new UNION operation.

Properties

Query

Gets the query to be combined with the base query.

UnionAll

Gets whether this is a UNION ALL operation (true) or a UNION operation (false).