Question

在下面的代码中，我需要检查每行中是否存在表。请告诉我怎么做。

Select * from Table 1 union all
select * from Table 2 union all
Select * from Table 3

我尝试了这个但是没有用

if objectid ('Table1') is not null
Select * from Table 1 union all
if objectid ('Table2') is not null
select * from Table 2 union all
if objectid ('Table3') is not null
Select * from Table 3

Answer 1

可能的解决方案是：

USE MyDB;

DECLARE @recordsExistingTables TABLE(Column1 nvarchar(50), Column2 nvarchar(50));

IF (EXISTS (SELECT * 
                 FROM INFORMATION_SCHEMA.TABLES 
                 WHERE TABLE_SCHEMA = 'dbo' 
                 AND  TABLE_NAME = 'Table1'))
BEGIN
   INSERT INTO @recordsExistingTables 
   SELECT *
   FROM Table1; 
END 

IF (EXISTS (SELECT * 
                 FROM INFORMATION_SCHEMA.TABLES 
                 WHERE TABLE_SCHEMA = 'dbo' 
                 AND  TABLE_NAME = 'Table2')) 
BEGIN
   INSERT INTO @recordsExistingTables 
   SELECT *
   FROM Table2; 
END 

IF (EXISTS (SELECT * 
                 FROM INFORMATION_SCHEMA.TABLES 
                 WHERE TABLE_SCHEMA = 'dbo' 
                 AND  TABLE_NAME = 'Table3')) 
BEGIN
   INSERT INTO @recordsExistingTables 
   SELECT *
   FROM Table3; 
END 


SELECT * FROM @recordsExistingTables;

使用表变量，只插入数据库中存在的表的行。在检查结束时，选择表变量的记录，您将拥有每个现有表的行。

Answer 2

尝试创建一种通用解决方案。创建了一个过程，您必须以逗号分隔值传递所有表名，它将返回所有表的完整数据列表。

Create Procedure spGetTableData
@tableNames varchar(4000) --expecting all table names in csv format here
As
Begin
declare @sql nvarchar(max);
declare @tablelist table(tablename varchar(50));

--getting all existint table names in one table
set @sql = 'select name from sys.objects where name in (''' + REPLACE(@tableNames, ',',''',''') + ''')';

insert into @tablelist
exec (@sql);

--creating query with union all
set @sql = '';
select @sql = @sql + 'Select * from ' + tablename + ' Union All ' From @tablelist;

set @sql = left(@sql, len(@sql) - 9);

exec sp_executesql @sql;

End

您可以执行以下操作：

Exec spGetTableData 'existing1,nonexisting1,existing2,existing3'

希望它有所帮助。

Answer 3

表不存在，因此您需要动态构建select语句。以下是您需要做的一个示例。我不会重现所有 if 语句，因为会有9种不同的组合，但你应该得到一般的想法。

    if objectid ('Table2') is not null AND objectid ('Table1') IS not null and objectid('Table3') is not null 
    @SelectStatement = 'Select * from Table1 UNION ALL SELECT * FROM TABLE2 UNION ALL SELECT * FROM TABLE3' 

    if objectid ('Table2') is not null AND objectid ('Table1') IS not null and objectid ('Table3') is null 
    @SelectStatement = 'Select * from Table1 UNION ALL SELECT * FROM TABLE2 ' 

    if objectid ('Table2') is not null AND objectid ('Table1') IS null and objectid ('Table3') is not null 
    @SelectStatement = 'Select * from Table2 UNION ALL SELECT * FROM TABLE3 ' 

    if objectid ('Table2') is null AND objectid ('Table1') IS NOT null and objectid ('Table3') is not null 
    @SelectStatement = 'Select * from Table1 UNION ALL SELECT * FROM TABLE3 ' 

    if objectid ('Table2') is null AND objectid ('Table1') IS null and objectid ('Table3') is not null 
    @SelectStatement = 'SELECT * FROM TABLE3 ' 

    if objectid ('Table2') is null AND objectid ('Table1') IS null and objectid ('Table3') is not null 
    @SelectStatement = 'SELECT * FROM TABLE3 ' 

//..... for 3 tables there will be 9 conbinations of TABLE1, TABLE2 and TABLE3 is null checks to perform and possible combinations

EXECUTE sp_executesql @FullStatement

Answer 4

值得注意的是，union要求所有列在类型上匹配，并且列数相同。

假设它们匹配（如果存在），并且您有固定数量的表，则以下查询有效：

declare @sql varchar(max)

if object_id('Table1') is not null
Begin
  set @sql = 'select * from Table1'
end 


if object_id('Table2') is not null
Begin
   if len(@sql) > 0 
   begin
      set @sql = @sql + char(10) + 'union all '
   end
  set @sql = @sql + 'select * from Table2'
end 


if object_id('Table3') is not null
Begin
   if len(@sql) > 0 
   begin
      set @sql = @sql + char(10) + 'union all '
   end
  set @sql = @sql + 'select * from Table3'
end 


execute(@sql)

首先创建一个动态SQL语句并将其存储在@SQL变量中。在完成评估/创建语句时，它使用execute（@sql）语句

执行

Answer 5

作为替代方案，我绝不建议这是最好的方法，但出于我自己的好奇心，我想知道CASE是否可以做到这一点。事实证明它可以。可能没有比任何其他解决方案更多或更少的笨重。

SET CONCAT_NULL_YIELDS_NULL OFF
DECLARE @sql NVARCHAR(4000)
SELECT @sql = CASE WHEN name = 'table1' THEN 'select * from table1 ' END FROM sys.tables WHERE name = 'table1'

SELECT @sql += CASE WHEN @sql IS NOT NULL AND name = 'table2' THEN ' union all select * from table2' WHEN @sql IS NULL AND name = 'table2' THEN 'select * from table2' END FROM sys.tables WHERE name = 'table2'

SELECT @sql += CASE WHEN @sql IS NOT NULL AND name = 'table3' THEN ' union all select * from table3' WHEN @sql IS NULL AND name = 'table3' THEN 'select * from table3' END FROM sys.tables WHERE name = 'table3'

PRINT @sql

EXEC sp_executesql @sql

你可以看到它工作on this Fiddle（除了sp_executesql位 - SQLFiddle不会返回结果）。

如果表存在于SQL Server 2008中，则为all all

5 个答案: