查詢 SQL SERVER 中某張表結構，sql 語句如下：
SELECT
tb.name AS tableName,
col.name AS columnName,
col.max_length AS length,
col.is_nullable AS isNullable,
t.name AS type,
(
SELECT
TOP 1 ind.is_primary_key
FROM
sys.index_columns ic
LEFT JOIN sys.indexes ind ON ic.object_id = ind.object_id AND ic.index_id= ind.index_id AND ind.name LIKE 'PK_%'
WHERE
ic.object_id = tb.object_id AND ic.column_id= col.column_id
) AS isPrimaryKey,
com.value AS comment
FROM
sys.TABLES tb
INNER JOIN sys.columns col ON col.object_id = tb.object_id
LEFT JOIN sys.types t ON t.user_type_id = col.user_type_id
LEFT JOIN sys.extended_properties com ON com.major_id = col.object_id
AND com.minor_id = col.column_id
WHERE
tb.name = '表名'
該 sql 可以正常執行，但是當把 sql 放到 jdbcTemplate 中執行時報一下錯誤：
Caused by: com.microsoft.sqlserver.jdbc.SQLServerException: The "variant" data type is not supported.
原因是 sql 語句 select 后面有 sql_variant 類型的屬性，在 JDBC 中不支持它。使用 sp_columns 命令最終查出 sys.extended_properties 表的 value 屬性的 TYPE_NAME 是 sql_variant 類型的，sql 如下：
sp_columns extended_properties
解決方法是使用 CONVERT 函數將該屬性轉成 varchar 類型。
CONVERT 函數的用法參考：SQL Server 中 CONVERT() 函數的使用。
修改后的 sql 語句為：
SELECT
tb.name AS tableName,
col.name AS columnName,
col.max_length AS length,
col.is_nullable AS isNullable,
t.name AS type,
(
SELECT
TOP 1 ind.is_primary_key
FROM
sys.index_columns ic
LEFT JOIN sys.indexes ind ON ic.object_id = ind.object_id AND ic.index_id= ind.index_id AND ind.name LIKE 'PK_%'
WHERE
ic.object_id = tb.object_id AND ic.column_id= col.column_id
) AS isPrimaryKey,
CONVERT(varchar(200), com.value) AS comment
FROM
sys.TABLES tb
INNER JOIN sys.columns col ON col.object_id = tb.object_id
LEFT JOIN sys.types t ON t.user_type_id = col.user_type_id
LEFT JOIN sys.extended_properties com ON com.major_id = col.object_id
AND com.minor_id = col.column_id
WHERE
tb.name = '表名'
參考：
com.microsoft.sqlserver.jdbc.SQLServerException: The "variant" data type is not supported.