How do I conditionally create a table in Sybase (TSQL)?

问题

OK, so Sybase (12.5.4) will let me do the following to DROP a table if it already exists:

IF EXISTS (
    SELECT 1
    FROM sysobjects
    WHERE name = 'a_table'
    AND type = 'U'
)
DROP TABLE a_table
GO

But if I try to do the same with table creation, I always get warned that the table already exists, because it went ahead and tried to create my table and ignored the conditional statement. Just try running the following statement twice, you'll see what I mean:

IF NOT EXISTS (
    SELECT 1
    FROM sysobjects
    WHERE name = 'a_table'
    AND type = 'U'
)
CREATE TABLE a_table (
    col1 int not null,
    col2 int null
)
GO

Running the above produces the following error:

SQL Server Error on (localhost) Error:2714 at Line:7 Message:There is already an object named 'a_table' in the database.

What's the deal with that?!

回答1:

The only workaround I've come up with so far is to use execute immediate:

IF NOT EXISTS (
    SELECT 1
    FROM sysobjects
    WHERE name = 'a_table'
    AND type = 'U'
)
EXECUTE("CREATE TABLE a_table (
    col1 int not null,
    col2 int null
)")
GO

works like a charm, feels like a dirty hack.

回答2:

There is no other way than calling create table in execute("create table ...")

SYBASE Manual says:

When a create table command occurs within an if...else block or a while loop, Adaptive Server creates the schema for the table before determining whether the condition is true. This may lead to errors if the table already exists. To avoid this situation, either make sure a view with the same name does not already exist in the database or use an execute statement, as follows:

if not exists
    (select * from sysobjects where name="my table")
begin
execute "create table mytable(x int)"
end

回答3:

I haven't tested this, but you could try moving the create table statement into a sproc. You could then conditionally call that sproc based on your existing if statement.

回答4:

Assign the "CREATE TABLE" statement in a char @variable and then do an EXEC(@variable).

回答5:

If you want to always create the table, but conditionally drop it, you can use:

IF(SELECT count(*) FROM sysobjects WHERE name="tableNameWithoutUserPart") > 0
    DROP TABLE tableNameWithUserPart
GO

CREATE TABLE tableNameWithUserPart ...

回答6:

There are no workarounds needed ;)

According to the documentation:

CREATE [ GLOBAL TEMPORARY ] TABLE [ IF NOT EXISTS ] [ owner.]table-name
( { column-definition | table-constraint | pctfree }, ... )
[ { IN | ON } dbspace-name ]
[ ENCRYPTED ]
[ ON COMMIT { DELETE | PRESERVE } ROWS
   | NOT TRANSACTIONAL ]
[ AT location-string ]
[ SHARE BY ALL ]

Just use the IF NOT EXISTS.

回答7:

IF object_id('a_table') IS NULL
BEGIN
    CREATE TABLE a_table (
        col1 int not null,
        col2 int null
    ) 
END

回答8:

This works when tested with Sybase anywhere 10.01 :

if not exists(select * from SysColumns where tname = 'AAA') then create table DBA.AAA(  UNIQUEID integer not null ) END IF ;

回答9:

Try using Begin and End.

IF NOT EXISTS ( SELECT Count(1) FROM sysobjects WHERE name = 'a_table' AND type = 'U' ) BEGIN CREATE TABLE a_table ( col1 int not null, col2 int null ) END GO

来源：https://stackoverflow.com/questions/307942/how-do-i-conditionally-create-a-table-in-sybase-tsql

标签

tsql

conditional

ddl

sybase-ase