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According to the SQL standards, a unique constraint on a single column means no two values are equal unless one of them is NULL.

Permission is granted to distribute this document without any alteration under the terms of the HSQLDB license.Many features of SQL92 and 99 up to Advanced Level are supported and there is support for most of SQL 2003 Foundation and several optional features of this standard.However, certain features of the Standards are not supported so no claim is made for full support of any level of the standards.Additional permission is granted to the HSQLDB Development Group to distribute this document with or without alterations under the terms of the HSQLDB license.HSQLDB 1.8.0 supports the dialect of SQL defined by SQL standards 92, .A unique constraint on multiple columns (c1, c2, c3, ..) means that no two sets of values for the columns are equal unless at lease one of them is NULL.Each single column taken by itself can have repeat values.With an index on one of the columns that is in the WHERE clause, it is often possible to start directly from the first candidate row and reduce the number of rows that are examined.Indexes are even more important in joins between multiple tables.HSQLDB does not currently prevent you from using a reserved word if it does not support its use or can distinguish it.For example BEGIN is a reserved words that is not currently supported by HSQLDB and is allowed as a table or column name.

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