SQL查询

Access数据库操作包括创建、查询、更新和删除数据，以及设置表间关系和用户权限等。

AS400数据库命令包括：CREATE、ALTER、DROP、SELECT、INSERT、UPDATE、DELETE等，用于创建、修改、删除和管理数据库对象。

不是acess数据库对象，可能是其他类型的数据库对象，如SQL Server、MySQL等。

AS在数据库中用于重命名列名或表名，使查询结果更易读。基本用法：SELECT column_name AS alias FROM table_name;

可以使用SELECT MAX(column_name) FROM table_name语句获取最大值，然后在CDM作业中使用该值进行操作。

在SQL中，可以使用LIMIT子句来查询每组的前几条记录。如果你有一个名为students的表，并且想要查询每个班级的前3名学生，你可以使用以下查询：，，``sql，SELECT * FROM students WHERE class = '计算机科学' ORDER BY score DESC LIMIT 3;，``

要查询连续登录的用户情况，可以使用以下SQL语句：，，``sql，SELECT user_id, MIN(login_time) AS start_time, MAX(login_time) AS end_time，FROM login_records，GROUP BY user_id，HAVING COUNT(*) ˃ 1;，``

GROUP BY用于对结果集进行分组，HAVING用于过滤分组后的结果。GROUP BY常与聚合函数一起使用，HAVING则用于设置条件。

要实现查询并自动创建缺失的索引，可以使用以下SQL语句：，，``sql，SELECT OBJECT_NAME(s.[object_id]) AS TableName,， i.name AS IndexName,， COL_NAME(ic.object_id,ic.column_id) AS ColumnName,， ic.index_id, ic.is_included_column,， USER_SEEKABLE(ic.index_id) AS IsSeekable,， ic.filter_definition，FROM sys.dm_db_missing_index_groups g，JOIN sys.dm_db_missing_index_group_stats m ON g.group_handle = m.group_handle，JOIN sys.dm_db_missing_index_details d ON m.index_handle = d.index_handle，JOIN sys.objects o ON d.object_id = o.object_id，JOIN sys.indexes i ON d.object_id = i.object_id AND d.index_id = i.index_id，JOIN sys.syscolumns c ON o.object_id = c.id AND c.colid = d.column_id，LEFT JOIN sys.partitions p ON o.object_id = p.OBJECT_ID AND i.index_id = p.index_id AND p.hobt_id = d.record_type，LEFT JOIN sys.allocation_units a ON p.partition_id = a.container_id，LEFT JOIN sys.system_internals s ON a.unit_id = s.allocunit_page_id，LEFT JOIN sys.dm_db_partition_stats ps ON p.object_id = ps.object_id AND ps.index_id = i.index_id AND ps.partition_number = p.partition_number，LEFT JOIN sys.dm_db_column_store_row_group csrg ON o.object_id = csrg.object_id AND csrg.columnstore_rowgroup_id = ps.partition_number，LEFT JOIN sys.dm_db_column_store_segments cs ON csrg.hobt_id = cs.hobt_id AND csrg.filegroup = cs.filegroup AND csrg.database_id = cs.database_id AND csrg.page_id = csrg.page_id，LEFT JOIN sys.dm_db_column_store_values ccv ON csrg.hobt_id = ccv.hobtid AND ccv.filegroup = csrg.filegroup AND ccv.database_id = csrg.database_id AND ccv.page_id = csrg.page_id AND ccvposition IS NOT NULL，LEFT JOIN sys.columns col ON o.object_id = col.object_id AND col.column_id = d.column_id，LEFT JOIN sys.computed_columns cc ON o.object_id = cc.object_id AND cc.column_id = d.column_id，WHERE i.is_primary_key = 0 AND i.is_unique = 1 AND i.is_ignored = 0 AND i.is_hypothetical = 0 AND i.has_filter = 0 AND i.allow_row_locks = 1 AND i.allow_page_locks = 1 AND i.preventive_maintenance = 0，ORDER BY gm.modification_counter DESC;，``