简单left join实现

JCalculator本身并没有提供left join这个接口，我实践中绝大多数都是在数据库就完成这步了。但数据库也有可能存在一个库是oracel,一个是mysql的情况，两个库同时用。一份数据从oracle出来，一份数据从mysql出来，无法关联。所有还是有必要给出一个这样的demo。

            
              // 商店ID关联数据
              var left = [
                { id: 1, name: "apple", price: 5, shopID: 2 },
                { id: 2, name: "banana", price: 3, shopID: 3 }
              ];

              var right = [
                { id: 1, name: "othersShop", tel: "1317866765" },
                { id: 2, name: "appleShop", tel: "1246531231" },
                { id: 3, name: "bananaShop", tel: "124512551" }
              ];

              //转成对象，类似数据库根据ID建立索引的功能，理解数据逻辑和性能上这都更好
              var rightIndex = {};

              jc.map(right, function (row) {
                rightIndex[row.id] = row
              })
              console.log(rightIndex)
              /* output: {
                 "1":{"id":1,"name":"othersShop","tel":"1317866765"},
                 "2":{"id":2,"name":"appleShop","tel":"1246531231"},
                 "3":{"id":3,"name":"bananaShop","tel":"124512551"}
                }*/
              
              var leftJoin = jc.sql({
                select: {
                  col: {
                    name: "name",
                    price: function (row) {
                      return row.price + "$";
                    },
                    shopName: function (row) {
                      var rightRow = rightIndex[row.shopID];
                      return rightRow.name;
                    },
                    tel: function (row) {
                      var rightRow = rightIndex[row.shopID];
                      return rightRow.tel;
                    }
                  }
                },
                from: left
              });
              console.log(leftJoin);
              /* output: [
                {"name":"apple","price":"5$","shopName":"appleShop","tel":"1246531231"},
                {"name":"banana","price":"3$","shopName":"bananaShop","tel":"124512551"}
              ]*/