I am stuck with this question. I know table can be created with the select statement.
CREATE TABLE new_orders (ordr_id,order_date DEFAULT SYSDATE,cus_id)
AS
Select order_id.order_date,customer_id FROM Orders;
Which statement would be true regarding the above question ?
The NEW_ORDERS table would not get created beacuse the default value cannot be specified in the column definition.
The NEW_ORDERS table would get created and only the NOT NULL constraint define on the specified columnns would be passed to the new table.
The NEW_ORDERS table would not get created because the column names in the CREATE TABLE command and the SELECT clause do not match.
The NEW_ORDERS table would get created and all the constraints defined on the specified columns in the orders table would be passed to new Table.
I think correct answer should be 3, but it's wrong. The correct answer according to my book is 2. I don't get it how?
The answer should not be 3. Because in the select statement, you are taking values from Orders table. So you will use the column names of that table. Selecting value from there, you are inserting it into new_orders. So, the column names don't have to be same because they are different tables.
Related
I am writing the following query to add a column at a specified position but getting the below error:
alter table quantum_raw_dev.rpt_backup_allocation
change upt_type upt_type STRING after tray_size;
You can add one or more columns to the end of the column list using:
ALTER TABLE <table_name> ADD COLUMNS (col_name col_type, ...);
[note: there is NO comma between column name and type]
Adding or Removing Columns
You can add one or more columns to the end of the column list using ADD COLUMNS,
or (with Impala only) you can delete columns using DROP COLUMN.
The general syntax is
ALTER TABLE tablename ADD COLUMNS (col1 TYPE1,col2 TYPE2,… );
ALTER TABLE tablename DROP COLUMN colname;
For example, you can add a bonus integer column to the employees table:
ALTER TABLE employees ADD COLUMNS (bonus INT);
Or you can drop the office_id column from the employees table:
ALTER TABLE employees DROP COLUMN office_id;
Notes
DROP COLUMN is not available in Hive, only in Impala. However, see “Replacing All Columns” below.
You can only drop one column at a time.
To drop multiple columns, use multiple statements or use the method to replace columns (see below).
You cannot add a column in the middle of the list rather than at the end.
You can, however, add the column then change the order (see above) or use the method to replace columns (see below).
As with changing the column order, these do not change the data files.
If the table definition agrees with the data files before you drop any column other than the last one,
you will need to recreate the data files without the dropped column's values.
If you drop the last column, the data will still exist but it will be ignored when a query is issued.
If you add columns for which no data exists, those columns will be NULL in each row.
Replacing All Columns
You can also completely replace all the columns with a new column list.
This is helpful for dropping multiple columns,
<h1>or if you need to add columns in the middle of the list<h1>
<h2>(like your use case)<h2>
The general syntax is
ALTER TABLE tablename REPLACE COLUMNS (col1 TYPE1,col2 TYPE2,… );
This completely removes the existing list of columns and replaces it with the new list.
Only the columns you specify in the ALTER TABLE statement will exist, and they will be in the order you provide.
Note
Again, this does not change the data files, only the metadata for the table,
so you'll either want the new list to match the data files or need to recreate the data files to match the new list.
I do not think you can add columns in between columns in Impala like above.
You can backup the data, drop the and recreate with new structure, and load the table from backup. Also if you have HIVE in your system you can try to do below steps -
Add column first and then use below commands to move columns around.
ALTER TABLE tab ADD COLUMNS (id BIGINT);
This moves id column to the beginning.
ALTER TABLE tab CHANGE COLUMN id id BIGINT first;
This moves existing_col after id.
ALTER TABLE tab CHANGE COLUMN existing_col existing_col string AFTER id;
Please refresh/invalidate metadata after applying all DDLs.
You cannot add column in between. Best way is to archive the data in another table. Drop the impala old table and create a fresh table with new columns as per the desired location and then reinsert the data.
I have a table with a 'user_id' column. Within that same table I have another data field labeled 'GMID'. Within that GMID column there are some fields that are null, the ones that aren't null have values that match the user_id data field within that row. Is there a way to create a script or query that will update all null fields in the GMID column to match the corresponding data values in the user_id row within that row? Are there any best practices I should follow, different approaches? Thanks in advance for anyone that can help.
Of course there is
UPDATE your_table
SET GMID=user_id
WHERE GMID IS NULL
But you even don't need WHERE if GMID always should be same as user_id.
By the way, why do you need two columns with same data in one table?
Another approach would be using the 'coalesce' function. It will return the first non-null value. This approach does not involve data changes on your table. On a query you can 'select coalesce(GMID, user_id) as GMID ...' it will return the first column that is not null.
Documentation for oracle DB:
http://docs.oracle.com/cd/B28359_01/server.111/b28286/functions023.htm
Update: I just reversed the name of the columns inside the coalesce function...
I want to copy the content of one column in table A and replace the contents (not insert into it - the number of rows will be the same) of another column in another table.
I can't a where condition, the table has only just been created at this point with one empty timestamp column. it will be populated via pyodbc class after the timestamps have been added - this query will fill the timestamps for me
What is the SQL command for this?
Thanks!
After discussion, this is the query needed : INSERT INTO OCAT_test_table (DateTimeStamp) SELECT DateTimeStamp FROM DunbarGen
I two tables, one table contains the names of columns, the other table contains the actual data that I need to display in a report. The data and columns are linked by a column key.
Is there a way to build a select statement that can use the column name from the column table for the actual temp table column?
Something like this;
Select A.value AS B.ColumnName
Simply put Can I create a table column name based on the value in another table in sql?
You can´t do it without using dynamic SQL as mentioned question that can be useful for you.
SQL Server: How to set column alias from another table during select?
I have a sql table that I am trying to add a column from another table to. Only when I execute the alter table query it does not pull the values out of the table to match the column where I am trying to make the connection.
For example I have column A from table 1 and column A from table 2, they are supposed to coincide. ColumnATable1 being an identification number and ColumnATable2 being the description.
I tried this but got an error...
alter table dbo.CommittedTbl
add V_VendorName nvarchar(200)
where v_venkey = v_vendorno
It tells me that I have incorrect syntax... Anyone know how to accomplish this?
alter table dbo.CommittedTbl
add V_VendorName nvarchar(200);
go
update c
set c.V_VendorName = a.V_VendorName
from CommittedTbl c
join TableA a
on c.v_venkey = a.v_vendorno;
go
I'm just guessing at your structure here.
alter table 2 add column A <some_type>;
update table2 set column A = (select column_A from table2 where v_venkey = v_vendorno);
Your names for tables and columns are a bit confusing but I think that should do it.
There is no WHERE clause for an ALTER TABLE statement. You will need to add the column (your first two lines), and then insert rows based upon a relationship you define between the two tables.
ALTER TABLE syntax:
http://msdn.microsoft.com/en-us/library/ms190273%28v=sql.90%29.aspx
There are several languages within SQL:
DDL: Data Definition Language - this defines the schema (the structure of tables, columns, data types) - adding a column to a table affects the table definitions and all rows will have that new column (not just some rows according to a criteria)
DML: Data Manipulation Language - this affects data within a table, and inserting, updating or other changes fall into this and you can update some data according to criteria (and this is where a WHERE clause would come in)
ALTER is a DDL statement, while INSERT and UPDATE are DML statements.
The two cannot really be mixed as you are doing.
You should ALTER your table to add the column, then INSERT or UPDATE the column to include appropriate data.
Is it possible that you want a JOIN query instead? If you want to join two tables or parts of two tables you should use JOIN.
have a look at this for a start if you need to know more LINK
hope that helps!