Question

对于给定的数据库数据结构：

Table      Attribute       Type                       Glossary

Species    Sp_name         C(10) P.K.                 Species name
           sp_woodtype     C(10)                      Wood Yielded by tree
           sp_maxht        I                          Max.height


Forest     Fo_name         C(10) P.K.                 Forest name
           Fo_size         I                          Forest area
           Fo loc          C(10)                      Geographical name
           Fo_comp         C(10)                      Forest owner


Tree       Tr_species      C(10) F.K. species.sp_name
           Tr_forest       C(10) F.K. forest.fo_name
           Tr_numb         I     P.K.                 Sequence number
           Tr_planted      Date                       Date of planting
           Tr_loc          C(10)                      Forest quadrant
           Tr_parent       I     F.K. tree.tr_numb    Procreating tree reference


Measure    Me_trnumb       I     F.K. tree.tr_numb
           Me_numb         I     P.K.                 Sequence number
           Me_result       I                          Test's measure
           Me_date         Date                       Measure taken on 
           Me_type         C(10)                      Type of measure

P.K。是主要的关键，F.K。是外键，C（N）字符（N）类型，I整数类型

我需要选择在所有森林中找到哪种树种，所以我尝试过以下方法，但似乎错了：

SELECT fo_name.forest, sp_name.species
FROM forest, species;

SELECT tr_species.tree, tr_forest.tree
FROM tree;

SELECT fo_name.forest, sp_name.species
FROM forest, species
INTERSECT
SELECT tr_species.tree, tr_forest.tree
FROM tree;

差异列表是一个比INTERSECT更好的解决方案来解决这个问题吗？

Answer 1

这是关系分工的一个特例您可以将每棵树的不同森林数量与森林总数进行比较，以找出：

SELECT tr_species
FROM   tree
GROUP  BY tr_species
HAVING count(DISTINCT tr_forest) = (SELECT count(*) FROM forest);

如果您需要的不仅仅是PK，请将结果加入表species。

顺便说一句，数据类型character(10)并不好，尤其不是PK列。

Any downsides of using data type "text" for storing strings?

多个表格中的差异列表与INTERSECT

1 个答案: