SQL#有用的SQL -- 1. COALESCE（sql使用什么语句）

#SQL#有用的SQL

-- 1. COALESCE: 返回列表中第一个非空值

SELECT COALESCE(column1, column2, 'default_value') AS result FROM table_name;

-- 2. NULLIF: 如果两个表达式相等，则返回 NULL，否则返回第一个表达式

SELECT NULLIF(column1, column2) AS result FROM table_name;

-- 3. CASE: 根据条件返回特定值的条件表达式

SELECT

CASE

WHEN condition1 THEN result1

WHEN condition2 THEN result2

ELSE default_result

END AS result

FROM table_name;

-- 4. ROLLUP: 生成小计和总计

SELECT department, SUM(salary)

FROM employees

GROUP BY department WITH ROLLUP;

-- 5. CUBE: 生成一个结果集，表示分组的所有可能组合

SELECT department, job_title, SUM(salary)

FROM employees

GROUP BY department, job_title WITH CUBE;

-- 6. EXISTS: 检查子查询中是否存在行

SELECT \* FROM Employees WHERE EXISTS (SELECT 1 FROM departments WHERE departments.id = employees.department_id);

-- 7. INTERSECT: 返回两个 SELECT 语句的公共记录

SELECT column_name FROM table1

INTERSECT

SELECT column_name FROM table2;

-- 8. EXCEPT: 返回第一个 SELECT 语句中不存在于第二个 SELECT 语句中的记录

SELECT column_name FROM table1

EXCEPT

SELECT column_name FROM table2;

-- 9. OFFSET-FETCH: 用于分页，跳过指定数量的行并返回下一组

SELECT \* FROM table_name ORDER BY column_name OFFSET 10 ROWS FETCH NEXT 5 ROWS ONLY;

-- 10. PIVOT: 将行转换为列以便于分析

SELECT \* FROM (SELECT Year, Quarter, Revenue FROM Sales) AS SourceTable

PIVOT (SUM(Revenue) FOR Quarter IN ([Q1], [Q2], [Q3], [Q4])) AS PivotTable;

-- 11. UNPIVOT: 将列转换为行

SELECT Year, Quarter, Revenue

FROM (SELECT Year, Q1, Q2, Q3, Q4 FROM Sales) AS SourceTable

UNPIVOT (Revenue FOR Quarter IN (Q1, Q2, Q3, Q4)) AS UnpivotedTable;

-- 12. 公共表表达式 (CTE): 可以在 SELECT、INSERT、UPDATE 或 DELETE 语句中引用的临时结果集

WITH CTE AS (

SELECT column1, column2 FROM table_name WHERE condition

)

SELECT \* FROM CTE;