如何使用 ROOM 格式化查询

Question

所以我有一个包含 4 列的 table，我想对 amount 列的值求和，其中 isExpense 是 true 并且 isExpense 是 false。我想减去这 2 个值和 return 那个总和。

除了单行查询外，我对 SQL 没有太多经验，所以我很难格式化它。

         @Query("""
                SELECT SUM (amount) AS INCOME FROM `transaction` WHERE isExpense = 0,
                SELECT SUM (amount) AS EXPENSE FROM `transaction` WHERE isExpense = 1,
                SUM (INCOME - EXPENSE) AS BALANCE
                """)
    fun getTotalBalance(): Flow<Double>?

如果所有其他方法都失败了，我可以通过在 table 中创建更多列来解决这个问题。

Answer 1

使用case表达式做条件聚合:

SELECT SUM(case when isExpense = 0 then amount else 0 end) AS INCOME,
       SUM(case when isExpense = 1 then amount else 0 end) AS EXPENSE,
       SUM(case when isExpense = 0 then amount
                when isExpense = 1 then -amount
           end) as BALANCE
FROM `transaction`

WHERE isExpense IN (0, 1) -- Not needed, but might speed things up if there
                          -- are other values than 0 and 1

如何使用 ROOM 格式化查询

How to format Query using ROOM

sql

android

kotlin

android-room