I am trying to add a new column in a monetDB database and I want it positioned after a specific one. In mysql this is possible using the AFTER keyword.
ALTER TABLE myTable ADD myNewColumn VARCHAR(255) AFTER myOtherColumn
I am trying this in the mclient:
sql>ALTER TABLE dbname.table_name ADD COLUMN new_name AFTER existing_name SET DEFAULT NULL;
What I get is a syntax error:
syntax error, unexpected AFTER in: "ALTER TABLE dbname.table_name ADD COLUMN new_name AFTER"
It is true that the ALTER documentation does not specify that AFTER exists, but I am hoping that anybody knows an alternative.
The safe way is to create an new table with the columns properly ordered and move the data; you probably know this already.
However if you really cannot do that, create a view:
CREATE VIEW AS SELECT [order the columns however you want here] FROM your_table;
Related
Working on hive table, where I need to change column name as below, its working as expected and changing column name but underline value of this column getting NULL.
ALTER TABLE db.tbl CHANGE hdfs_loaddate hdfs_load_date String;
Here changed column name is hdfs_load_date and values are getting NULL after renaming column name.
Does any one have idea to fix this. Thanks in advance!!
#Ajay_SK Referencing this article: Hive Alter table change Column Name
There is a comment:
Note that the column change will not change any underlying data if it is a parquet table. That is, if you have data in the table already, renaming a column will not make the data in that column accessible under the new name: select a from test_change; 1 alter table test_change change a a1 int; select a1 from test_change; null
He is specific to parquet, but the scenario you describe is similar where you have successfully changed the name, but hive still thinks the original data is in the original key.
A better approach to solve your issue, would be to create a new table of the schema you want with column name change. Then perform an Insert INTO new table select FROM * old table.
I want to add a new column to a specific location in hive table. when i add new column it goes to the last position.
You need to recreate table. If the table is external and data already contains new column, then issue drop and create table statements. General solution is to:
1. create new_table...;
2. insert overwrite new_table select from old_table;
3. drop old_table;
4. alter new_table rename to old_table;
Also if datafiles already contain new column in some position you can
1. Alter table add column
Change column position using this example:
2. ALTER TABLE test_change CHANGE old_name new_name STRING AFTER other_col CASCADE;
See docs here: Change Column Name/Type/Position/Comment
How frequently are people running SELECT *?? Typically, people list out each column in the select statement. Just add the column to the end, and adjust like SELECT last_col, first_col, second_col ...
Alternatively, create a VIEW that runs a select statement with the column ordering you want.
Rename the table to something else, and name the view to the table, and no one would know any different
I've a very complex table structure in hive, let's say that it's like the following table:
create table dirceu ( a struct<b:string,c:string>);
Now I do need to add another subcolumn to the a column, and it should have the structure b,c and d, I'm trying to do it with the following alter table:
alter table dirceu change column a a struct<b:string,c:string, d:string>;
But this throw the following error:
Error: Error while processing statement: FAILED: Execution Error, return code 1 from org.apache.hadoop.hive.ql.exec.DDLTask. Unable to alter table. The following columns have types incompatible with the existing columns in their respective positions :
a (state=08S01,code=1)
Is there a way to do this using alter table? I know that I can do it using create table and copy the data, but I would like to know if there is another way to do it.
UPDATE
I'm using hive: 2.1.0.2.6.1.0-129
HortonWorks: HDP-2.6.1.0
This is the command you need to give,
ALTER TABLE dirceu CHANGE COLUMN a a STRUCT<b:STRING, c:STRING, d:STRING>;
You can't add a new field to Collection data type. Instead, you need to completely change the schema.
Hope you like the answer. Yippee!!
I guess it works by this command,
alter table dirceu add column a struct<d:string>;
Please let me know if it doesn't work by comment.
im trying to add a new column to my existing table 'Results', and it seems to be very easy but I cant see the mistake.
Here is my code:
SQL> Alter table results add column CAL ENUM('A','B');
ERROR at line 1: ORA-00904: : invalid identifier
What am I missing?
I've read this from w3 and this from java2s but cant see the difference to mine.
Thanks, and sorry for the dumb question.
OK, with an ORA- error I am assuming that this is an oracle database, and not mysql. you have both tags and you are linking to MySQL example, but the error is not a MySQL error.
Assuming that this IS an Oracle DB, then there is no native ENUM data type. You have to do this as follows: First - you add the column with a correctly defined data type, and second you create a constrained list of permitted values on that column as a check constraint.
Alter table results add (cal varchar2(1));
Alter table results add constraint chk_cal CHECK (cal in ('A','B')) ENABLE;
(EDITED to fix brackets in constraint creation line)
How can I change DATA TYPE of a column from number to varchar2 without deleting the table data?
You can't.
You can, however, create a new column with the new data type, migrate the data, drop the old column, and rename the new column. Something like
ALTER TABLE table_name
ADD( new_column_name varchar2(10) );
UPDATE table_name
SET new_column_name = to_char(old_column_name, <<some format>>);
ALTER TABLE table_name
DROP COLUMN old_column_name;
ALTER TABLE table_name
RENAME COLUMN new_column_name TO old_coulumn_name;
If you have code that depends on the position of the column in the table (which you really shouldn't have), you could rename the table and create a view on the table with the original name of the table that exposes the columns in the order your code expects until you can fix that buggy code.
You have to first deal with the existing rows before you modify the column DATA TYPE.
You could do the following steps:
Add the new column with a new name.
Update the new column from old column.
Drop the old column.
Rename the new column with the old column name.
For example,
alter table t add (col_new varchar2(50));
update t set col_new = to_char(col_old);
alter table t drop column col_old cascade constraints;
alter table t rename column col_new to col_old;
Make sure you re-create any required indexes which you had.
You could also try the CTAS approach, i.e. create table as select. But, the above is safe and preferrable.
The most efficient way is probably to do a CREATE TABLE ... AS SELECT
(CTAS)
alter table table_name modify (column_name VARCHAR2(255));
Since we can't change data type of a column with values, the approach that I was followed as below,
Say the column name you want to change type is 'A' and this can be achieved with SQL developer.
First sort table data by other column (ex: datetime).
Next copy the values of column 'A' and paste to excel file.
Delete values of the column 'A' an commit.
Change the data type and commit.
Again sort table data by previously used column (ex: datetime).
Then paste copied data from excel and commit.