Question

我想执行SELECT，只有在表中存在该列时才会选择列值，否则显示为null。

这就是我目前正在做的事情：

SELECT TOP 10 CASE WHEN EXISTS
    (SELECT * FROM INFORMATION_SCHEMA.COLUMNS 
    WHERE TABLE_SCHEMA ='test' and TABLE_NAME='tableName' and COLUMN_NAME='columnName') 
THEN columnName ELSE NULL END AS columnName

我也试过这个：

SELECT TOP 10 CASE WHEN 
    (SELECT COUNT(*) FROM INFORMATION_SCHEMA.COLUMNS 
    WHERE TABLE_SCHEMA ='test' and TABLE_NAME='tableName' and COLUMN_NAME='columnName') >0 
THEN columnName ELSE NULL END AS columnName

如果列中存在列，它们都可以正常工作。但是当列不存在时，它会给我错误：

列名称无效＆＃39; columnName＆＃39;

Answer 1

你可以写成：

SELECT CASE WHEN EXISTS
(
    SELECT * FROM INFORMATION_SCHEMA.COLUMNS 
    WHERE TABLE_SCHEMA ='test' and TABLE_NAME='tableName' 
    and COLUMN_NAME='columnName'
) 
THEN
(
    SELECT COLUMN_NAME FROM INFORMATION_SCHEMA.COLUMNS 
    WHERE TABLE_SCHEMA ='test' and TABLE_NAME='tableName'
    and COLUMN_NAME='columnName'
)
ELSE
NULL 
END
AS columnName

DEMO

编辑：如果您希望从表的列中选择前10个值（如果该列存在），则需要将动态查询编写为：

SELECT @columnVariable =     
CASE WHEN EXISTS
(
    SELECT * FROM INFORMATION_SCHEMA.COLUMNS
    WHERE TABLE_SCHEMA ='test' and TABLE_NAME='tableName' 
    and COLUMN_NAME='columnName'
) 
THEN
(
    SELECT COLUMN_NAME FROM INFORMATION_SCHEMA.COLUMNS
    WHERE TABLE_SCHEMA ='test' and TABLE_NAME='tableName' 
    and COLUMN_NAME='columnName'
)
ELSE
NULL 
END


/* Build the SQL string one time.*/
SET @SQLString =
     N'SELECT TOP 10 ' + @columnVariable+ '
       FROM test.tableName ';      


EXECUTE sp_executesql @SQLString

DEMO2

Answer 2

试试这个：

SELECT
    TOP 1
        CASE 
            WHEN COLUMN_NAME='columnName' THEN 
                COLUMN_NAME 
            ELSE 
                NULL 
            END 
        AS COLUMN_NAME
FROM        
    INFORMATION_SCHEMA.COLUMNS 
WHERE
    TABLE_SCHEMA ='test' and TABLE_NAME='tableName'
ORDER BY
    COLUMN_NAME DESC

Answer 3

SELECT *
        FROM sys.columns
        WHERE [name] = N'columnName'
            AND [object_id] = OBJECT_ID(N'tableName')

在case语句中添加它。请注意，此代码仅适用于更高版本的sqlserver。[如sqlserver 2008]

SQL Server如果仅在表中存在列时才选择列

3 个答案: