Php 当使用大小写时，如果在左联接中找不到匹配行，则忽略列

我想拿一份医院账单的收款报告。有一张表格记录了病人支付的预付款。并不是所有的病人都预付

这就是我现在使用的查询

"SELECT SUM(
    CASE 
    WHEN bill_type = 3 THEN b.payable_amount + w.balance
    ELSE 0
 END) AS 'amount'

 FROM bills b 
    LEFT JOIN ip_patients ip on ip.id = b.ip_id
    LEFT JOIN advance w on w.id = (SELECT x.id FROM advance x WHERE x.ip_id = ip.id ORDER BY x.created_at DESC  LIMIT 1)
    WHERE b.doctor_id = '$d->id' AND CAST(b.created_at AS DATE) >= '$date1' AND CAST(b.created_at AS DATE) <= '$date2' AND b.is_cancelled = 0 AND b.is_deleted = 0"

---

如果患者未支付预付款，则预付款表中不会插入任何行。上述查询的结果是，当预付病人表中没有行时，他的付款将被完全忽略，即账单表中的值不会添加到总和中。

如果将null添加到not null，则结果为null。比如说

MariaDB [sandbox]> set @a = null;
Query OK, 0 rows affected (0.00 sec)

MariaDB [sandbox]> set @b = 100;
Query OK, 0 rows affected (0.00 sec)

MariaDB [sandbox]>
MariaDB [sandbox]> select @a+@b amt;
+------+
| amt  |
+------+
| NULL |
+------+
1 row in set (0.00 sec)

您可以使用ifnull来处理这个问题

set @a = null;
set @b = 100;

select ifnull(@a,0) + ifnull(@b,0) amt;

+-----+
| amt |
+-----+
| 100 |
+-----+
1 row in set (0.00 sec)

---

如果将null添加到not null，则结果为null。比如说

MariaDB [sandbox]> set @a = null;
Query OK, 0 rows affected (0.00 sec)

MariaDB [sandbox]> set @b = 100;
Query OK, 0 rows affected (0.00 sec)

MariaDB [sandbox]>
MariaDB [sandbox]> select @a+@b amt;
+------+
| amt  |
+------+
| NULL |
+------+
1 row in set (0.00 sec)

您可以使用ifnull来处理这个问题

set @a = null;
set @b = 100;

select ifnull(@a,0) + ifnull(@b,0) amt;

+-----+
| amt |
+-----+
| 100 |
+-----+
1 row in set (0.00 sec)