MySQL教程之技巧

三、技巧
 
1、1=1,1=2的使用,在SQL语句组合时用的较多
 
“where 1=1” 是表示选择全部    “where 1=2”全部不选,
如:
if @strWhere !=”
begin
set @strSQL = ‘select count(*) as Total from [‘ + @tblName + ‘] where ‘ + @strWhere
end
else
begin
set @strSQL = ‘select count(*) as Total from [‘ + @tblName + ‘]’
end
 
我们可以直接写成
 
错误!未找到目录项。
set @strSQL = ‘select count(*) as Total from [‘ + @tblName + ‘] where 1=1 安定 ‘+ @strWhere 2、收缩数据库
–重建索引
DBCC REINDEX
DBCC INDEXDEFRAG
–收缩数据和日志
DBCC SHRINKDB
DBCC SHRINKFILE
 
3、压缩数据库
dbcc shrinkdatabase(dbname)
 
4、转移数据库给新用户以已存在用户权限
exec sp_change_users_login ‘update_one’,’newname’,’oldname’
go
 
5、检查备份集
RESTORE VERIFYONLY from disk=’E:\dvbbs.bak’
 
6、修复数据库
ALTER DATABASE [dvbbs] SET SINGLE_USER
GO
DBCC CHECKDB(‘dvbbs’,repair_allow_data_loss) WITH TABLOCK
GO
ALTER DATABASE [dvbbs] SET MULTI_USER
GO
 
7、日志清除
SET NOCOUNT ON
DECLARE @LogicalFileName sysname,
@MaxMinutes INT,
@NewSize INT
 
USE tablename — 要操作的数据库名
SELECT  @LogicalFileName = ‘tablename_log’, — 日志文件名
@MaxMinutes = 10, — Limit on time allowed to wrap log.
@NewSize = 1  — 你想设定的日志文件的大小(M)
 
Setup / initialize
DECLARE @OriginalSize int
SELECT @OriginalSize = size
FROM sysfiles
WHERE name = @LogicalFileName
SELECT ‘Original Size of ‘ + db_name() + ‘ LOG is ‘ +
CONVERT(VARCHAR(30),@OriginalSize) + ‘ 8K pages or ‘ +
CONVERT(VARCHAR(30),(@OriginalSize*8/1024)) + ‘MB’
FROM sysfiles
WHERE name = @LogicalFileName
CREATE TABLE DummyTrans
(DummyColumn char (8000) not null)
 
DECLARE @Counter    INT,
@StartTime DATETIME,
@TruncLog   VARCHAR(255)
SELECT @StartTime = GETDATE(),
@TruncLog = ‘BACKUP LOG ‘ + db_name() + ‘ WITH TRUNCATE_ONLY’
 
DBCC SHRINKFILE (@LogicalFileName, @NewSize)
EXEC (@TruncLog)
— Wrap the log if necessary.
WHILE @MaxMinutes > DATEDIFF (mi, @StartTime, GETDATE()) — time has not expired
AND @OriginalSize = (SELECT size FROM sysfiles WHERE name = @LogicalFileName)
AND (@OriginalSize * 8 /1024) > @NewSize
BEGIN — Outer loop.
SELECT @Counter = 0
WHILE   ((@Counter < @OriginalSize / 16) AND (@Counter < 50000))
BEGIN — update
INSERT DummyTrans VALUES (‘Fill Log’) DELETE DummyTrans
SELECT @Counter = @Counter + 1
END
EXEC (@TruncLog)
END
SELECT ‘Final Size of ‘ + db_name() + ‘ LOG is ‘ +
CONVERT(VARCHAR(30),size) + ‘ 8K pages or ‘ +
CONVERT(VARCHAR(30),(size*8/1024)) + ‘MB’
FROM sysfiles
WHERE name = @LogicalFileName
DROP TABLE DummyTrans
SET NOCOUNT OFF
 
8、说明:更改某个表
exec sp_changeobjectowner ‘tablename’,’dbo’
 
9、存储更改全部表
 
CREATE PROCEDURE dbo.User_ChangeObjectOwnerBatch
@OldOwner as NVARCHAR(128),
@NewOwner as NVARCHAR(128)
AS
 
DECLARE @Name    as NVARCHAR(128)
DECLARE @Owner   as NVARCHAR(128)
DECLARE @OwnerName   as NVARCHAR(128)
 
DECLARE curObject CURSOR FOR
select ‘Name’    = name,
‘Owner’    = user_name(uid)
from sysobjects
where user_name(uid)=@OldOwner
order by name
 
OPEN   curObject
FETCH NEXT FROM curObject INTO @Name, @Owner
WHILE(@@FETCH_STATUS=0)
BEGIN
if @Owner=@OldOwner
begin
set @OwnerName = @OldOwner + ‘.’ + rtrim(@Name)
exec sp_changeobjectowner @OwnerName, @NewOwner
end
— select @name,@NewOwner,@OldOwner
 
FETCH NEXT FROM curObject INTO @Name, @Owner
END
 
close curObject
deallocate curObject
GO
 
10、SQL SERVER中直接循环写入数据
declare @i int
set @i=1
while @i<30
begin
insert into test (userid) values(@i)
set @i=@i+1
end
案例
有如下表,要求就裱中所有沒有及格的成績,在每次增長0.1的基礎上,使他們剛好及格:
 
    Name     score
 
    Zhangshan   80
 
    Lishi       59
 
    Wangwu      50
 
    Songquan    69
 
while((select min(score) from tb_table)<60)
 
begin
 
update tb_table set score =score*1.01
 
where score<60
 
if  (select min(score) from tb_table)>60
 
  break
 
 else
 
    continue
 
end
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