mysql使用JOIN表进行多次查询并将结果合并到1行

Question

SELECT save AS 'Simpanan', owe AS 'Hutang'
FROM
(
SELECT t.amount AS save, NULL AS owe
FROM trans t JOIN category cat
ON t.category_id = cat.category_id
WHERE cat.category_id = 1
UNION ALL
SELECT NULL AS save, t.amount AS owe
FROM trans t JOIN category cat
ON t.category_id = cat.category_id
WHERE cat.category_id = 2
)

我一直在努力解决这个 mysql 查询。我想要实现的是，结果变成只有 1 行。目前此查询输出 2 行，其中列中的某些字段为 NULL。我怎样才能让它只输出 1 行，如下所示:

Simpanan    Hutang  
500         200

看看这里:http://sqlfiddle.com/#!5/89973/30

Answer 1

如果您确定只有一个结果，请使用子查询。

SELECT 
   (SELECT t.amount AS save
    FROM trans t JOIN category cat
    ON t.category_id = cat.category_id
    WHERE cat.category_id = 1) AS 'Simpanan',
    (SELECT t.amount AS owe
    FROM trans t JOIN category cat
    ON t.category_id = cat.category_id
    WHERE cat.category_id = 2) AS 'Hutang'

或者:

SELECT MAX(save) AS 'Simpanan', MAX(owe) AS 'Hutang'
FROM
(
SELECT t.amount AS save, NULL AS owe
FROM trans t JOIN category cat
ON t.category_id = cat.category_id
WHERE cat.category_id = 1
UNION ALL
SELECT NULL AS save, t.amount AS owe
FROM trans t JOIN category cat
ON t.category_id = cat.category_id
WHERE cat.category_id = 2
)