To solve these problems, the SEQ and PREDATOR systems introduce a spe- cial sublanguage, called SEQUINfor queries on sequences [Seshadri et al. 1994, 1995; Seshadri 1998]. SEQUIN works on sequences in combination with SQL working on standard relations; query blocks from the two languages can be nested inside each other, with the help of directives for converting data be- tween the blocks. SEQUIN’s special algebramakes the optimization of sequence queries possible, but optimization between sequence queries and set queries is not supported; also its expressive power is still too limited for many application areas. To address these problems, SRQL [Ramakrishnan et al. 1998] augments relational algebra with a sequential.
