带有两个不同表和条件的SELECT sql语句

Question

我的数据库中有两个表，就像

部

//Department
D#      DNAME
---------------
1      RESEARCH
2        IT
3       SCIENCE

项目

//Project
Budget      D#
---------------
22500       1
22300       1
50000       2

所以我用这句话

SELECT D#,DNAME and sum(budget) as total budget
    FROM DEPARTMENT,PROJECT
GROUP BY D#;

我的输出应该是，如果一个部门没有任何项目预算，则平均值显示为0

D#      DNAME      total budget
-------------------------------
1       RESEARCH      44800
2          IT         50000
3        SCIENCE        0

Answer 1

您缺少where子句：

SELECT d.D#,DNAME and sum(budget) as total budget
FROM DEPARTMENT d, PROJECT p
where d.d# = p.dt#
GROUP BY d.D#;

或包括没有预算的部门：

SELECT d.D#,DNAME and sum(budget) as total budget
FROM DEPARTMENT d
left outer join PROJECT p
on d.d# = p.d#
GROUP BY d.D#;

Answer 2

这可能对您有帮助

   SELECT `D#`,`DNAME`, 
   IFNULL((SELECT sum(`budget`) 
          FROM `project` WHERE `project.`D#` = `Department`.`D#` ),0) AS `total budget` 
   FROM `Department`

如果对此查询执行EXPLAIN ，您可能会注意到，与使用GROUP BY查询相比，它使用索引的效率更高