Sql Oracle联合取决于IF条件

我必须对所有这些语句进行SQL查询：

首先，我有第一个SQL查询，如下所示：

SELECT COUNT(*) as NbOfEntries from table1 where colName = "some condition"

SELECT 
  t2.col1 as col1, 
  t2.col2 as col2, 
  t3.col3 as col3 
FROM table2 t2 
  INNER JOIN table3 t3 
  ON t2.colName = t3.colName 
WHERE t2.colName = "some condition"

SELECT 
  "Dumb string 1" as col1,
  "Dumb string 2" as col2, 
  "Dumb string 3" as col3
FROM DUAL -- or something like that, I don't know how it works

col1    col2  col3
A       AB    3
A       AC    7
A       AD    2
X       XX    0 // << Add this last row

然后，我有另一个SQL语句，如下所示：

SELECT COUNT(*) as NbOfEntries from table1 where colName = "some condition"

SELECT 
  t2.col1 as col1, 
  t2.col2 as col2, 
  t3.col3 as col3 
FROM table2 t2 
  INNER JOIN table3 t3 
  ON t2.colName = t3.colName 
WHERE t2.colName = "some condition"

SELECT 
  "Dumb string 1" as col1,
  "Dumb string 2" as col2, 
  "Dumb string 3" as col3
FROM DUAL -- or something like that, I don't know how it works

col1    col2  col3
A       AB    3
A       AC    7
A       AD    2
X       XX    0 // << Add this last row

最后，基于第一条SQL语句，如果

NbOfEntries>0

，我必须在第二条语句的结果上添加一行伪数据。我必须向第二个SQL查询添加一个

UNION

，如下所示：

SELECT COUNT(*) as NbOfEntries from table1 where colName = "some condition"

SELECT 
  t2.col1 as col1, 
  t2.col2 as col2, 
  t3.col3 as col3 
FROM table2 t2 
  INNER JOIN table3 t3 
  ON t2.colName = t3.colName 
WHERE t2.colName = "some condition"

SELECT 
  "Dumb string 1" as col1,
  "Dumb string 2" as col2, 
  "Dumb string 3" as col3
FROM DUAL -- or something like that, I don't know how it works

col1    col2  col3
A       AB    3
A       AC    7
A       AD    2
X       XX    0 // << Add this last row

我曾想过使用

UNION

，但我不知道如何在有条件的情况下使用

UNION

。此外，我不知道如何在Oracle中使用

SELECT

生成假数据，我已经了解了

DUAL

的工作原理，但我还没有真正能够向前推进

多谢各位

编辑：样本数据

如果第一条语句返回0，则有以下行：

col1    col2  col3
A       AB    3
A       AC    7
A       AD    2

如果第一条语句返回>0，我必须向结果中添加一个伪行，如下所示：

SELECT COUNT(*) as NbOfEntries from table1 where colName = "some condition"

SELECT 
  t2.col1 as col1, 
  t2.col2 as col2, 
  t3.col3 as col3 
FROM table2 t2 
  INNER JOIN table3 t3 
  ON t2.colName = t3.colName 
WHERE t2.colName = "some condition"

SELECT 
  "Dumb string 1" as col1,
  "Dumb string 2" as col2, 
  "Dumb string 3" as col3
FROM DUAL -- or something like that, I don't know how it works

col1    col2  col3
A       AB    3
A       AC    7
A       AD    2
X       XX    0 // << Add this last row

col1 col2 col3
A AB 3
A AC 7
A广告2
X XX 0/您可以这样使用
EXISTS
：

SELECT 
  t2.col1 as col1, 
  t2.col2 as col2, 
  t3.col3 as col3 
FROM table2 t2 
  INNER JOIN table3 t3 
  ON t2.colName = t3.colName 
WHERE t2.colName = "some condition"
UNION ALL
SELECT 
  "Dumb string 1" as col1,
  "Dumb string 2" as col2, 
  "Dumb string 3" as col3
FROM DUAL
WHERE EXISTS (
  SELECT 1 from table1 
  where colName = "some condition"
)

无需对
表1
的行进行计数

EXISTS
如果有一行满足条件，将返回
TRUE
。
您可以使用
HAVING
子句进行筛选：

SELECT t2.col1 as col1, t2.col2 as col2, t3.col3 as col3 
FROM table2 t2 
INNER JOIN table3 t3 ON t2.colName = t3.colName 
WHERE t2.colName = 'some condition'
UNION ALL
SELECT 'X', 'XX', 0 
FROM table1 
WHERE colName = 'some condition'
HAVING COUNT(*) > 1

请向我们展示样本数据和预期结果，以澄清您的问题。@GMB我已更新了问题