Q: 102 A SELECT statement can be used to perform these three functions:
1. Choose rows from a table.
2. Choose columns from a table.
3. Bring together data that is stored in different tables by creating a link between them.
Which set of keywords describes these capabilities?
A. difference, projection, join
B. selection, projection, join
C. selection, intersection, join
D. intersection, projection, join
E. difference, projection, product
Answer: B
问下，这个题目意思我懂了，但是difference、projection、selection、intersection、intersection都是干嘛的啊？不是sql关键字又不是sql术语啊，怎么识别呢？
------解决方案--------------------------------------------------------
这个属于关系型数据库理论知识。
Selection：选择。意思是从表中查询出部分行。
Projection：投影。意思是从表中查询出部分列。
intersection:交集。多表的共同部分。
difference：同上就是不同部分。。