从两个表中选择的SQL查询 - 如果另一个表中没有表，则从一个表返回结果

Question

我正在运行此SQL查询：

SELECT a.retail, b.cost 
from call_costs a, call_costs_custom b 
WHERE a.sequence = b.parent 
 AND a.sequence = '15684' 
 AND b.customer_seq = '124'

如果行存在于call_costs_custom ，则返回a.retail和b.cost但如果该行不存在，我想使用WHERE子句显示a.retail a. (call_costs) a. (call_costs)

Answer 1

来自W3Schools：

LEFT JOIN关键字返回左表（table1）中的所有行，右表（table2）中的匹配行。 没有匹配时，右侧的结果为NULL。

SELECT 
    a.retail,
    b.cost 
FROM 
    call_costs a
LEFT JOIN 
    call_costs_custom b 
        ON 
    a.sequence = b.parent 
        AND
    b.customer_seq = '124'
WHERE 
    a.sequence = '15684' 

Answer 2

您需要一个外部联接，即即使第二个表中没有匹配项也会保留第一个表中的记录的联接。 使用LEFT OUTER JOIN或短LEFT JOIN因此：

select cc.retail, ccc.cost 
from call_costs cc
left join call_costs_custom ccc on ccc.parent = cc.sequence and ccc.customer_seq = '124'
where cc.sequence = '15684';

Answer 3

你的解释不是100％明确，但这是我的尝试：

SELECT COALESCE(b.cost,a.retail)
FROM call_costs a
LEFT JOIN call_costs_custom b 
ON a.sequence = b.parent 
  AND b.customer_seq = '124'
WHERE a.sequence = '15684'