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Tham khảo tài liệu 'managing time in relational databases- p19', công nghệ thông tin, cơ sở dữ liệu phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 346 Chapter 14 ALLEN RELATIONSHIP AND OTHER QUERIES AND c.eff_beg_dt cl.row_crt_dt AND c.eff_end_dt cl.row_crt_dt AND c.asr_beg_dt cl.row_crt_dt AND c.asr_end_dt cl.row_crt_dt WHERE cl.claim_amt p.copay_amt ORDER BY cl.adjud_dt c.client_nbr p.policy_nbr p.eff_beg_dt To conclude this section we show what this query might look like if the SQL language supported PERIOD datatypes and also our taxonomy of Allen relationships. We suppose that the taxonomynode fills 1 is represented by the reserved word INCLUDES. With a SQL language like this the Asserted Versioning schema no longer has pairs of dates to represent its two time periods. Instead it has the single columns asr_per and eff_per. SELECT c.client_nbr c.client_nm p.policy_nbr p.policy_type p.copay_amt cl.service_dt cl.claim_amt cl.adjud_dt FROM Claim cl INNER JOIN Policy_AV p ON p.policy_oid cl.policy_oid AND p.eff_per INCLUDES cl.service_dt AND p.asr_per INCLUDES cl.adjud_dt INNER JOIN Client_AV c ON c.client_oid p.client_oid AND c.eff_per INCLUDES cl.row_crt_dt AND c.asr_per INCLUDES cl.row_crt_dt WHERE cl.claim_amt p.copay_amt ORDER BY cl.adjud_dt c.client_nbr p.policy_nbr p.eff_beg_dt In either form what is striking about the query is its simplicity relative to the complexity of the bi-temporal semantics that underlies it. Unlike queries in the standard temporal model and for that matter uni-temporal queries in the alternative temporal model as well this query does not assemble a collection of rows and then proceed to check for temporal gaps and temporal overlaps within sub-selected collections of those rows. Asserted Versioning enforces bi-temporal semantics once as the data is being created and modified rather than each time the data is queried. In Other Words With appropriate temporal extensions to the SQL language the expression of all thirteen Allen relationships and of this and other relationships which are combinations of those Chapter 14 ALLEN RELATIONSHIP AND OTHER QUERIES 347 thirteen relationships .
