如何选择有条件的数据

Question

我正在尝试检索某个日期范围内的所有数据行，其中只有一个条件，即仅检索“余额”类型的第一行，我认为我的问题应该用数据样本更好地解释。

这是我的表格声明

| date | type     | credit | debit | balance |
|   a  | balance  |        |       |  100.00 |
|   a  | credit   | 50.00  |       |         |
|   a  | credit   | 50.00  |       |         |
|   a  | debit    |        | 50.00 |         |
|   b  | balance  |        |       |  50.00  |
|   b  | credit   | 50.00  |       |         |
|   b  | credit   | 50.00  |       |         |
|   b  | credit   | 50.00  |       |         |
|   b  | balance  |        |       | -100.00 |
|   c  | debit    |        | 250.00|         |

这就是我想要做的（预期结果）

我需要执行一个查询，返回从日期 a 到 c 的所有行，但只返回类型为 balance 的第一行

| date | type     | credit | debit | balance |
|   a  | balance  |        |       |  100.00 |
|   a  | credit   | 50.00  |       |         |
|   a  | credit   | 50.00  |       |         |
|   a  | debit    |        | 50.00 |         |
|   b  | credit   | 50.00  |       |         |
|   b  | credit   | 50.00  |       |         |
|   b  | credit   | 50.00  |       |         |
|   c  | debit    |        | 250.00|         |

我在这里知道的是我可以通过执行 2 个不同的查询来实现

SELECT * FROM statement WHERE date >= a and date <= c and type = 'balance' ORDER BY date ASC LIMIT 1
SELECT * FROM statement WHERE date >= a and date <= c and type != 'balance' ORDER BY date ASC

这是我的问题，出于某种原因，我需要使用 1 个单一查询来完成此操作，我如何通过一个查询来实现此结果。

Answer 1

您尝试过 UNION 吗？

SELECT * FROM statement WHERE date >= a and date <= c and type = 'balance' ORDER BY date ASC LIMIT 1
UNION
SELECT * FROM statement WHERE date >= a and date <= c and type != 'balance' ORDER BY date ASC

Answer 2

将Union 与distinct 一起使用

SELECT * FROM statement WHERE date >= a and date <= c and type = 'balance' ORDER BY date ASC LIMIT 1
UNION DISTINCT
SELECT * FROM statement WHERE date >= a and date <= c and type != 'balance' ORDER BY date ASC

Answer 3

我认为您不需要同时提供type = 'balance' 和type != 'balance' 条件。这没有任何意义。以下查询将获取相同的记录。

SELECT * 
FROM statement 
WHERE date >= a and date <= c  ORDER BY type,date ASC