Lambda Expression으로 Join사용하기

Lambda Expression에서 기본적인 Join은 다음과 같은 구분을 따릅니다. Key에 해당하는 정보가 양쪽 Table에 존재하는 경우에만 조회됩니다.

구문형식

var result = 
    sourceTable
    .Join
        <SourceTableType,
        JoinedTableType, 
        MappingKeyType, 
        ResultType>
        (joinedTables, 
        sourceTableParameter => sourceTableParameter.mappingKeyColumn, 
        joinedTableParameter => joinedTableParameter.mappingKeyColumn,
        (sourceTableParameter, joinedTableParameter) => new ResultType
        {
            ....
        });

예제

IEnummerable<StudentsInUniversity> result = 
    universities.Join
                <University, 
                Student, 
                long, 
                StudentsInUniversity>
                (students, 
                u => u.UniversityId, 
                s => s.UniversityId,
                (u, s) => new StudentsInUniversity
                {
                    StudentName = s.Name,
                    UniversityName = u.Name
                });

Left Outer Join은 다음과 같은 다른 구문을 가집니다.

구문형식

var result = 
    sourceTable
    .GroupJoin(joinedTable,
                sourceTableParameter => sourceTableParameter.mappingKeyColumn,
                joinedTableParameter => joinedTableParameter.mappingKeyColumn,
                (sourceTableParameter, joinedTableParameter) => new { sourceTable = sourceTableParameter, joinedTable = joinedTableParameter})
    .SelectMany(
        t  => t.joinedTable.DefaultIfEmpty(),
        (groupedList, joinedTalbe) => new
        {
            ....
        });

예제

IEnummerable<StudentsInUniversity> result = 
    universities
    .GroupJoin(students,
                u => u.UniversityId,
                s => s.UniversityId,
                (u, s) => new { universities = u, students = s })
    .SelectMany(
        t  => t.students.DefaultIfEmpty(),
        (grouped, s) => new StudentsInUniversity
        {
            UniversityName = grouped.unversities.Name,
            StudentName = s.Name,
        });