为什么我的SQL查询提取的行多于表中的行？

Question

salary_sheet表中有35行，但是此查询获取30660行，不知道为什么？

 function queryCheck() {
  $sql = "select  salary_sheet.payment_period_id,salary_sheet.employee_id,salary_sheet.supervisorwise_serial,"
."salary_sheet.employee_type_id,salary_sheet.sl,salary_sheet.cl,salary_sheet.el,salary_sheet.basic,salary_sheet.hr,"
."salary_sheet.medical,salary_sheet.other,salary_sheet.total,salary_sheet.min_wages,"
."salary_sheet.prod_wages,salary_sheet.attn_bonus,salary_sheet.prod_bonus,"
."salary_sheet.ot_hours,salary_sheet.ot_rate,salary_sheet.ot_amount,salary_sheet.stamp_deduct,"
."salary_sheet.absent_deduction,salary_sheet.net_payable,"

."employee.employee_code,employee.employee_name,employee.designation_id,employee.employee_grade,"

."attendance_summery.working_days "
."from salary_sheet "
."left join employee on salary_sheet.employee_id = employee.employee_id  "
."left join attendance_summery on salary_sheet.payment_period_id = attendance_summery.payment_period_id    "
."where salary_sheet.employee_type_id = 9 and salary_sheet.payment_period_id = 41  "             
."order by salary_sheet.supervisorwise_serial desc";
$query = $this->db->query($sql);
$rows = $query->num_rows();
return $rows;

 }

有人的帮助表示赞赏。

Answer 1

这是您的查询。

select  salary_sheet.payment_period_id,
salary_sheet.employee_id,
salary_sheet.supervisorwise_serial,
salary_sheet.employee_type_id,salary_sheet.sl,salary_sheet.cl,salary_sheet.el,salary_sheet.basic,salary_sheet.hr,
salary_sheet.medical,salary_sheet.other,salary_sheet.total,salary_sheet.min_wages,
salary_sheet.prod_wages,salary_sheet.attn_bonus,salary_sheet.prod_bonus,
salary_sheet.ot_hours,salary_sheet.ot_rate,salary_sheet.ot_amount,salary_sheet.stamp_deduct,
salary_sheet.absent_deduction,salary_sheet.net_payable,
employee.employee_code,employee.employee_name,employee.designation_id,employee.employee_grade,
attendance_summery.working_days
from salary_sheet
left join employee 
on salary_sheet.employee_id = employee.employee_id
left join attendance_summery 
on salary_sheet.payment_period_id = attendance_summery.payment_period_id
where salary_sheet.employee_type_id = 9 and salary_sheet.payment_period_id = 41             
order by salary_sheet.supervisorwise_serial desc

第一个“左联接”在左边包含employee_type_id 9和payment_period_id 41的工作表行。 所有这些工作表都将保留。 在左侧联接的右侧 ，所有行都会出现，无论这些行是否存在于这些员工（ 这就是左侧联接所做的事情 ）。 因此，如果存在它们，它们就来；如果不存在，它们就为空值。

第一步的结果是在左联接上的表，对表Attenance_summary进行相同的操作。

因此，是的，当您无意中犯了一个错误时，左联接至少可以起到重复作用。