CMPUT 291 2022_09_19

CMPUT 291 2022_09_19

sql and 关系代数

EG.找到薪水大于50k的人的名字与电话

SELECT name, phone 
FROM emp
WHERE salary > 50000

//RC

{<e.name,e.phone>|emp(e)^e.salaet > 50000}

RA

RC

• domin : set of relations
• operators:
  • basic: selection prseject cartesian product renaming union set diff
  • join set intersection division
• some features:
  • good to describe evaluation steps
  • can optimze the steps