Question

我有使用courseID，studentName的表，另一个表是courseID，courseName用户将根据学生姓名在学生表中搜索课程名称我应该使用什么sql语法？加入或内部联接

Answer 1

你可以试试这个：

$(function () {
  var reasons = [
    { Id: 1, Reason: "Late", SafeName: "late" },
    { Id: 2, Reason: "Road Works", SafeName: "road_works" },
    { Id: 3, Reason: "Later", SafeName: "later" },
  ]

  var history = [
    { Name: "John", Team: "Team1" },
    { Name: "Peter", Team: "Team1" },
    { Name: "Simon", Team: "Team2" }
  ]

  function GetData(){
    return [history, reasons];
  }

  function ReasonModel(data, parent) {
    var self = this;
    ko.mapping.fromJS(data, {}, parent.Reasons)
  }

  function DelayModel(data, parent) {
var self = this;
    ko.mapping.fromJS(data, {}, parent.History)
  }

  function ViewModel() {
    var self = this;

    var mapping = {
      reasons: {
        create: function (options) {
          return new ReasonModel(options.data, self);
        }
      },
      history: {
        create: function (options) {
          return new DelayModel(options.data, self);
        }
      },
    }

    self.History = ko.observableArray([]);
    self.Reasons = ko.observableArray([]);
    self.SelectedReason = ko.observable();

    self.GetHistory = function(){
      GetData().done(function(result){
        ko.mapping.fromJS(result, mapping, self);
      })
    }
  }
  var vm = new ViewModel();
  ko.applyBindings(vm);
  vm.GetHistory();
});

Answer 2

如果您使用JOIN并指定约束（例如on a.courseId = b.courseId），则与您使用INNER JOIN完全相同，因此根据您的表格结构，您应该执行以下操作：

select * from studentTable
inner join courseTable 
on studentTable.courseID = courseTable.courseID
where studentTable.studentName = 'Jack Black';

在MySQL中你也可以写

select * from studentTable, courseTable 
where studentTable.courseID = courseTable.courseID
and studentName = 'Jack Black';

因为内部会以同样的方式查询。

在此处查看联接的确切语法：http://dev.mysql.com/doc/refman/5.7/en/join.html

Answer 3

您在寻找联合结果时加入，例如选择所有学生姓名及其所有课程。 E.g：

select s.studentname, c.coursename
from course c
join student_takes_course s on s.courseid = c.courseid;

或（因为表的顺序并不重要）：

select s.studentname, c.coursename
from student_takes_course s
join course c on s.courseid = c.courseid;

JOIN只是INNER JOIN的缩写。你可以使用。

但是当你对加入的结果不感兴趣时，那么不加入就是一个好习惯。例如，当您只想显示课程表中的课程名称时，如您的示例所示。然后，您通常会使用IN或EXISTS：

select coursename 
from course
where courseid in 
(
  select courseid
  from student_takes_course
  where studentname = 'Joe'
);

或

select c.coursename 
from course c
where exists
(
  select *
  from student_takes_course s
  where s.studentname = 'Joe'
  and s.courseid = c.courseid
);

（我更喜欢IN条款优先于EXISTS条款，并尽可能使用它们。）

在MySQL中我可以将id FK与另一个表中的varchar进行比较

3 个答案: