apache calcite 进阶篇（一）

介绍

上一篇已经介绍过如何利用SQL解析查询出hello world，通过工厂模式实例化Scheam, schema包含表，表包含数据。没看过的可以去看https://blog.csdn.net/qq_27408211/article/details/86497097

用过工具的基本也都是这种形态。今天学习用不一样的方式也非常简单的代码实现复杂的SQL解析。

代码结构

pom.xml

    4.0.0
    com.calcite
    calcite-test
    1.0-SNAPSHOT
    calcite-test

    
        UTF-8
        1.8
        1.8
    

    
        
        
            org.apache.calcite
            calcite-core
            1.18.0
        
    

讲解

相信大家都见过以下代码

这是一段连接jdbc数据的代码，需要在pom文件引入mysql驱动。换成calcite之后。变成

Client01.java

package com.calcite2;

import org.apache.calcite.adapter.java.ReflectiveSchema;
import org.apache.calcite.jdbc.CalciteConnection;
import org.apache.calcite.schema.SchemaPlus;

import java.sql.*;

public class Client01 {

    /**
     * //        SchemaFactory factory = new ReflectiveSchema.Factory();
     * //        final ImmutableMap.Builder builder = ImmutableMap.builder();
     * //        builder.put("class","com.calcite2.FoodMart");
     * //        builder.put("staticMethod","instance");
     * //        Map operand = builder.build();
     * //        Schema schema = factory.create(rootSchema,"hr",operand);
     * //        rootSchema.add("hr",schema);
     * */
    public static void main(String[] args) throws ClassNotFoundException, SQLException {
        Class.forName("org.apache.calcite.jdbc.Driver");
        Connection connection = DriverManager.getConnection("jdbc:calcite:lex=JAVA");
        CalciteConnection calciteConnection = connection.unwrap(CalciteConnection.class);
        final SchemaPlus rootSchema = calciteConnection.getRootSchema();
        rootSchema.add("hr", new ReflectiveSchema(new HrSchema()));
        calciteConnection.setSchema("hr");  // 设置默认Schema
        Statement statement = calciteConnection.createStatement();
        ResultSet resultSet = statement.executeQuery(
                "select d.deptno, min(e.empid) from hr.emps as e join hr.depts as d on e.deptno = d.deptno " +
                        "where e.deptno = 10 group by d.deptno having count(*) > 1");
        ResultSetMetaData metaData = resultSet.getMetaData();
        int columnCount = metaData.getColumnCount();
        int lineIndex = 1;
        while(resultSet.next()){
            System.out.println("行序号 => " + lineIndex);
            for (int i = 1; i < columnCount + 1; i++) {
                Object value = resultSet.getObject(i);
                System.out.println(String.format("\t列序号 => %s, 值 => %s, 类型 => %s", i,value,metaData.getColumnTypeName(i)));
            }
            lineIndex += 1;
        }
        resultSet.close();
        statement.close();
        connection.close();
    }
}

以上代码着重点

上面的代码最重要的就是 

new ReflectiveSchema(new HrSchema()) 这段代码可以看源代码是通过反射实例化Schema中的字段，设置hr数据库。

calciteConnection.setSchema("hr");  // 设置默认Schema，如果没有这句，每次写SQL都要指明hr.表名。

 

HrSchema.java

没错，就是模拟两个表的之前的关系。数据结构如图

接下来我们的任务是 查出部门编号为10并且至少有1个员工部门编号和最小员工ID

Employee.java

package com.calcite2;

public class Employee {
    public final int empid;
    public final int deptno;
    public final String name;
    public final float salary;
    public final Integer commission;

    public Employee(int empid, int deptno, String name, float salary, Integer commission) {
        this.empid = empid;
        this.deptno = deptno;
        this.name = name;
        this.salary = salary;
        this.commission = commission;
    }

    @Override public boolean equals(Object obj) {
        return obj == this
                || obj instanceof Employee
                && empid == ((Employee) obj).empid;
    }

    public String toString() {
        return "Employee [empid: " + empid + ", deptno: " + deptno
                + ", name: " + name + "]";
    }
}

Department.java

package com.calcite2;

import java.util.List;

public class Department {
    public final int deptno;
    public final String name;
    
    @org.apache.calcite.adapter.java.Array(component = Employee.class)
    public final List employees;

    public Department(int deptno, String name, List employees) {
        this.deptno = deptno;
        this.name = name;
        this.employees = employees;
    }

    @Override public boolean equals(Object obj) {
        return obj == this
                || obj instanceof Department
                && deptno == ((Department) obj).deptno;
    }

    public String toString() {
        return "Department [deptno: " + deptno + ", name: " + name
                + ", employees: " + employees + "]";
    }
}

一对多数据关系一定要加上@org.apache.calcite.adapter.java.Array(component = Employee.class) 这句，不然SQL会报空指针

 

如上代码就可以完成稍微复杂的关系查询了如：

select d.deptno, min(e.empid) 
    from 
        hr.emps as e 
        join 
        hr.depts as d 
    on e.deptno = d.deptno
where e.deptno = 10 
group by d.deptno 
having count(*) > 1

两个表关联查询结果：