如何从SQL中的三个表中检索数据 [英] How to retrieve data from three tables in SQL

问题描述

我有这张桌子

table1                    table2                          table3
---------------          ----------------------------          ---------------------
NO.    courses           NO.   id        courses    grade          NO.       courses
---------------          -----------------------------         ---------------------
1      CHEM 101          1    2255      CHEM 101    A+             2         English    
2      ENGL 101          2    2255      English     A
3      MATH 101     

我如何编写 SQL 查询以从以前的表中按 id 检索数据，以便给我这样的表，其中它们之间的关系为 NO.

How I can write SQL query to retrieve data by id from previous tables in order to give me like this table where the relation between them is NO.

----------------------------------------
course     grade     equal of my courses
----------------------------------------
CHEM 101   A+            null
ENGL 101   A             English   
MATH 101   null          null

推荐答案

你应该使用 LEFT JOIN 用于.我假设 NO 是维护所有表之间关系的列名

You should use LEFT JOIN for. I assume NO is a column name that maintain the relationship between all tables

SELECT t1.courses,t2.grade,t3.courses AS 'equal of my courses'
FROM table1 t1
LEFT JOIN table2 t2
ON t1.NO = t2.NO
LEFT JOIN table3 t3
ON t2.NO = t3.NO;

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