sql - 如果查询返回一行，如何选择一条记录，或者如果查询返回更多行，如何选择无记录？

Question

如果只有一行存在，我需要选择一行，如果有更多行，它应该选择 0 行。

Answer 1

如果您使用的是 PL/SQL，那么如果返回多于一行，则选择使用的列select-into将引发异常：too_many_rows

declare
  var table.column%type;
begin
  select column
  into   var
  from   table
  where  ...;
end;

如果您只想使用 SQL 来执行此操作，那么您可以执行以下操作：

select *
from
  (select s.*, count(*) over () c
   from
    (select *
     from table
     where ...
     and   rownum <= 2
    ) s
)
where c = 1

更新

正如 DazzaL 在评论中所说，rownum <= 2限制的原因是如果结果集中有超过 2 行，则将查询短路。如果数据集很大，这可以带来显着的性能优势。

Answer 2

我想出了这个，只是为了它，使用 CTE

With counter as
( select count(any_field) as cnt from your_query
)
SELECT
  your_query
WHERE exists (SELECT cnt from Counter WHERE cnt=1)

有 1 条记录时为 1 行 - http://sqlfiddle.com/#!4/84c7b/2
超过 1 条记录时为 0 行 - http://sqlfiddle.com/#!4/95c4a/1

编辑
或者如果你想避免重复整个查询......一个例子：（
使用来自 sqlfiddle http://sqlfiddle.com/#!4/6a2d8/117的模式）

With results as
( select * from montly_sales_totals
),
counter as
( SELECT count(name) as cnt FROM results
)
SELECT *
FROM results
WHERE exists (SELECT cnt from Counter WHERE cnt=5)

Answer 3

SELECT fld1, fld2
FROM (SELECT COUNT(*) over() cnt ,fld1, fld2 FROM tbl WHERE fld1 = 'key')
WHERE cnt = 1

Answer 4

如果只有一行存在，我需要选择一行，如果有更多行，它应该选择 0 行。

我假设该表仅包含您有兴趣查看（或不查看）的行，在这种情况下，我会写类似

select *
  from table1
 where 1 = (select count(1) 
              from table1
           )

如果您只想从表的结果子集中查看一行，我会选择类似的内容：

with t as ( select *
              from table1
             where [put here your condition]
)
select *
  from t
 where 1 = (select count(1) 
              from t
           )

Answer 5

试试这个：

SELECT f1,f2
FROM  Table
WHERE (f1 = @f1) AND (f2=@f2) AND (f3=@f3)
GROUP BY f1,f2
HAVING (COUNT(*) = 1)

Answer 6

试试这个：

SELECT col1, col2 FROM
(SELECT count(id) as 'cnt', col1, col2 FROM table_name WHERE col1='value') 
WHERE cnt=1;

Answer 7

声明 COL_COUNT NUMBER；
BEGIN
COL_COUNT: = 0 ;
SELECT COUNT (1) INTO COL_COUNT FROM USER_TAB_COLUMNS WHERE TABLE_NAME = '(你的表名)';
IF COL_COUNT = 0 THEN
EXECUTE IMMEDIATE ('select * from dual') ;
万一;
结尾;