在SQL领域，试题是检验和提升技能的有效方式。以下是对提供的SQL试题的详细解答：
1. 检索出test_a表中各个id段的年龄合计值，且年龄合计值大于50：
```sql
SELECT ID, SUM(YAGE) AS TotalAge
FROM TEST_A
GROUP BY ID
HAVING SUM(YAGE) > 50;
```
2. 检索出test_a表中所有字段信息，同时添加两个字段，显示同一id内的人数和同一年龄的人数：
```sql
SELECT a.*, COUNT(*) OVER (PARTITION BY ID) AS SameIdCount,
COUNT(*) OVER (PARTITION BY ID, YAGE) AS SameAgeCount
FROM TEST_A a;
```
3. 检索出test_a表中每个id范围内年龄最大的前两名的人员的所有信息：
```sql
WITH MaxAgeRank AS (
SELECT *, RANK() OVER (PARTITION BY ID ORDER BY YAGE DESC) AS AgeRank
FROM TEST_A
)
SELECT *
FROM MaxAgeRank
WHERE AgeRank <= 2;
```
4. 检索出test_a表中的记录，其id在test_b表中出现过2次以上（含2次）：
```sql
SELECT a.*
FROM TEST_A a
WHERE EXISTS (
SELECT 1
FROM TEST_B b
WHERE a.ID = b.ID
GROUP BY b.ID
HAVING COUNT(*) >= 2
);
```
5. 检索出test_a表中年龄比test_b表平均年龄大的记录总数及平均年龄：
```sql
WITH AvgAgeB AS (
SELECT AVG(YAGE) AS AvgAge
FROM TEST_B
)
SELECT COUNT(*) AS CountOfRecords, AVG(a.YAGE) AS AvgAgeInA
FROM TEST_A a
JOIN AvgAgeB ON 1=1
WHERE a.YAGE > AvgAgeB.AvgAge;
```
6. 统计A表中每个月注册的用户数、显示月份和统计的数目，以及统计A表中有姓名相同的用户数、显示姓名和统计的数目：
```sql
-- 统计每个月注册的用户数
SELECT TO_CHAR(REGDATE, 'YYYY-MM') AS Month, COUNT(*) AS UserCount
FROM A
GROUP BY TO_CHAR(REGDATE, 'YYYY-MM');
-- 统计姓名相同的用户数
SELECT NAME, COUNT(*) AS NameCount
FROM A
GROUP BY NAME
HAVING COUNT(*) > 1;
```
如果需要将相同的记录写入B表：
```sql
INSERT INTO B (ID, NAME)
SELECT ID, NAME
FROM (
SELECT ID, NAME
FROM A
GROUP BY ID, NAME
HAVING COUNT(*) > 1
) duplicates;
```
然后保留A表中注册时间最大的记录：
```sql
DELETE FROM A
WHERE ID NOT IN (
SELECT ID
FROM (
SELECT ID, MAX(REGDATE) AS MaxRegDate
FROM A
GROUP BY ID
) max_dates
WHERE A.REGDATE = max_dates.MaxRegDate
);
```
7. 检索出表A与表B通过CODE关联，同一CODE时只检索出TIME最大的记录：
```sql
SELECT A.*
FROM A
JOIN (
SELECT CODE, MAX(TIME) AS MaxTime
FROM A
GROUP BY CODE
) max_times ON A.CODE = max_times.CODE AND A.TIME = max_times.MaxTime;
```
8. 取出STATUS全部为9的ID：
```sql
SELECT ID
FROM A
WHERE STATUS = 9
GROUP BY ID
HAVING COUNT(*) = (SELECT COUNT(*) FROM A);
```
9. 根据企业名称、欠费金额、是否欠费展示表格，这里需要具体的表结构和字段名称，但基本SQL逻辑是筛选和聚合。
10. 展示oid、owner、num之和（Sum_num）以及status状态：
```sql
WITH StatusCounts AS (
SELECT oid, owner, SUM(num) AS Sum_num,
CASE
WHEN COUNT(CASE WHEN status = 1 THEN 1 END) = COUNT(*) THEN '完成'
WHEN COUNT(CASE WHEN status = 2 THEN 1 END) > 0 THEN '完成中'
ELSE '初始'
END AS S_status
FROM YourTable
GROUP BY oid, owner
)
SELECT * FROM StatusCounts;
```
11. 求出id中按序号升序排列后的后2条数据：
```sql
WITH SortedScores AS (
SELECT id, score, ROW_NUMBER() OVER (PARTITION BY id ORDER BY 序号) AS RowNum
FROM YourTable
)
SELECT id, 序号, score
FROM SortedScores
WHERE RowNum IN (2, 3)
ORDER BY id, 序号;
```
12. 查询重叠时间段：
```sql
WITH OverlappingPeriods AS (
SELECT Employee_sn, StartDate, EndDate,
LAG(EndDate, 1, StartDate) OVER (PARTITION BY Employee_sn ORDER BY StartDate) AS PrevEndDate
FROM YourTable
)
SELECT a.Employee_sn, a.StartDate, GREATEST(a.EndDate, b.EndDate) AS EndDate
FROM OverlappingPeriods a
JOIN OverlappingPeriods b ON a.Employee_sn = b.Employee_sn AND a.StartDate < b.EndDate AND a.PrevEndDate > b.StartDate;
```
这些SQL语句展示了数据库查询的各种常见技巧，包括聚合函数、分组、窗口函数、子查询、连接操作以及处理日期和时间等。通过练习这些题目，可以加深对SQL语言的理解和应用能力。
