如果联接字段值在另一个表中不存在，则从JOIN查询中获取默认值

Question

我有两个表customers和reviews 。 结构是这样的：

顾客：-

|---------------------------------|
|  id  |         name             |
|---------------------------------|
|   1  |       Thutmekri          |
|---------------------------------|
|   3  |        Conan             |
|---------------------------------|

评论： -

|-------------------------------------------|
|   id   |   business_id  |    customer_id  |
|-------------------------------------------|
|   1    |       1        |        1        |
|-------------------------------------------|
|   2    |       1        |        2        |
|-------------------------------------------|
|   3    |       1        |        3        |
|-------------------------------------------|

reviews的customer是customer_id的id 。

联接查询，

SELECT customers.name, reviews.id as review_id 
FROM customers, reviews 
WHERE customers.id = reviews.customer_id

返回这样的数据集：

|----------------------------------|
|    review_id  |      name        |
|----------------------------------|
|       1       |      Thutmekri   |
|----------------------------------|
|       3       |      Conan       |
|----------------------------------|

但我希望它返回：

|----------------------------------|
|    review_id  |      name        |
|----------------------------------|
|       1       |      Thutmekri   |
|----------------------------------|
|       2       |      N/A         |
|----------------------------------|
|       3       |      Conan       |
|----------------------------------|

对于评论中的customer_id（客户表中没有任何数据），我希望显示“ N / A”。 我该怎么做？

以下查询也无济于事

SELECT reviews.id as review_id, COALESCE( name, 'N/A' )
FROM customers, reviews
WHERE customers.id = reviews.customer_id

Answer 1

使用左联接并切换联接中表的顺序：

SELECT
    r.id AS review_id,
    COALESCE(c.name, 'N/A') AS name
FROM reviews r
LEFT JOIN customers c
    ON c.id = r.customer_id;

SQLFiddle

Answer 2

尝试这个 ：

SELECT r.id  as review_id, COALESCE( c.name, 'N/A' ) FROM reviews r
LEFT JOIN customers c ON c.id = r.customer_id

Answer 3

使用右外部联接可实现此目的：

SELECT
    reviews.id as review_id,
    COALESCE(name, 'N/A' )
FROM customers
RIGHT JOIN reviews
    ON customers.id = reviews.customer_id;

Answer 4

您可以使用以下查询

选择R.id AS review_id，COALESCE（C.name，'N / A'）AS名称来自评论R LEFT JOIN客户C ON R.customer_id = C.id;