把不是连号的数据找出来

有一组不连号的数据，把中间不连号的列出来

--生成测试表及测试数据
CREATE TABLE tb(col1 varchar(10),col2 int)
INSERT tb SELECT 'a',2
UNION ALL SELECT 'a',3
UNION ALL SELECT 'a',6
UNION ALL SELECT 'a',7
UNION ALL SELECT 'a',8
UNION ALL SELECT 'b',1
UNION ALL SELECT 'b',5
UNION ALL SELECT 'b',6
UNION ALL SELECT 'b',7
GO

--缺号分布查询
SELECT a.col1,start_col2=a.col2+1,
    end_col2=(
        SELECT MIN(col2) FROM tb aa
        WHERE col1=a.col1 AND col2>a.col2 
            AND NOT EXISTS(
                SELECT * FROM tb WHERE col1=aa.col1 AND col2=aa.col2-1))
        -1
FROM(
    SELECT col1,col2 FROM tb
    UNION ALL --为每组编号补充查询起始编号是否缺号的辅助记录
    SELECT DISTINCT col1,0 FROM tb
)a,(SELECT col1,col2=MAX(col2) FROM tb GROUP BY col1)b
WHERE a.col1=b.col1 AND a.col2<b.col2 --过滤掉每组数据中,编号最大的记录
    AND NOT EXISTS(
        SELECT * FROM tb WHERE col1=a.col1 AND col2=a.col2+1)
ORDER BY a.col1,start_col2

--查询结果
col1       start_col2  end_col2    
-------------- -------------- ----------- 
a          1           1
a          4           5
b          2           4