选择、投影、连接(国外英文资料).docVIP

选择、投影、连接(国外英文资料) Selection, projection, and connection operations A, choose Choice is also called restriction, which is the group of elements that choose to satisfy a given condition in the relationship R. Student: sigma f (R) = {t | t, t, f (t) = true} The F represents the selection condition, which is a logical expression that takes the logical value true or false. The basic form of the logical expression F is X1, theta Y1 [phi X2 theta Y2]... Theta, which means the comparison operator, can be , greater than or equal, less than or equal to or not. X1, Y1, etc. Are the attribute names or constants or simple functions. The property name can also be replaced with its serial number. Theta logical operators, may be Sunday afternoon or ∨, etc. [] the appointment. The parts of the [] may or may not be. ... This format can be repeated. So the choice operation is actually picking from the relationship R to make the logical expression F the real tuple, which is the operation from the Angle of the row. This is shown in figure 2-5 (a). Figure 2-5 Lets give an example. There are student relationship student, Course relationship Course and elective relationship SC, as shown in figure 2-6 below, all of the following examples are the operations on these three relationships. Figure 2-6 【 1 】 the inquiry information system (IS department) all the students. Sigma Sdept = Is (Student), which Is shown in figure 2-7 (a). Figure 2-7 【 2 】 the query is less than 20 years old. Sigma Sage 20 (Student), the result is shown in figure 2-7 (b). Second, the projection The projection onto R is a new relationship that selects several properties columns from the R. Remember: Its A (R) A (R) = {t [A]. Where A is the property column in R. The projection operation is an operation from the Angle of the column, as shown in figure 2-5 (b). Query Student relationship Students projection on both the Students name and their respective properties. The results are shown in figure 2-8 (a), which is shown in