TOSQL developed by Ariav (1986) is an extension of SQL. It allows access to both current data and their previous versions, but does not include update semantics. Snodgrass (1987) extended Quel to TQuel with language constructs to retrieve facts that have been time stamped with a validity interval. HTQUEL (Homogeneous Temporal Quel) proposed by Gadia (1988) is based on the same query language as TQuel. Time is considered as discrete equidistant time intervals. Kim et al. (1990) developed ETQL as a front-end system to INGRES. ETQL supports abstract time (including relative time, e.g., last spring) to achieve easier specification of time in queries. In 1994 the specification of TSQL2 was published (Snodgrass et al., 1994) which later resulted in proposals for an extension to SQL3 called SQL3/temporal.
Standard SQL does not include time support except for user-defined time. Neither transaction time nor valid time is available (see Table 2.2). Date and time support in SQL-92 are similar to that in DB2 (Melton and Simon, 1993). The design for SQL3 only corrected some of the inconsistencies, but contains no additional temporal support over SQL-92 (Pissinou et al., 1994). However, until 1994 the SQL3 proposals included several constructs that can be useful for temporal extensions (e.g., interval data type). Since then several change proposals were submitted to the ANSI and ISO SQL3 standards committees for adding a new part termed SQL/Temporal (Snodgrass, 1997; Snodgrass et al., 1996b,a).
None of these temporal query languages are built for complex spatial queries. Oracle recently announced their support for the HHCODE standard for spatial attributes. Such efforts could induce further developments in the research of temporal query languages including spatial aspects.
Commercial database systems often support transaction time mechanism based on tuple time-stamping. Valid time is not supported as a built-in functionality.