I am working on this assignment question and it is asking me:
To create a table called (TEMP_CUST) from an existing table Customers
View the content and constraints of TEMP_CUST table
What I have done so far is I have created my table, didn't add any constraints to the table TEMP_CUST and viewed the table using the DESC command.
Here is the code for table creation
CREATE TABLE TEMP_CUST
AS
(SELECT
CUSTOMER#, LASTNAME,
FIRSTNAME, ADDRESS, CITY,
STATE, ZIP, REFERRED,
REGION, EMAIL
FROM
CUSTOMERS);
DESC TEMP_CUST;
Now that I have done that I want to view the constraints of the table. I have used this command but am not sure if it is correct.
SELECT *
FROM USER_CONSTRAINTS
WHERE TABLE_NAME = 'TEMP_CUST';
i have used this command, but not sure if it is correct.
You haven't said why you don't think it's correct so we have to guess the reason for your doubt. Perhaps it's because the set of constraints you get is smaller than the set of constraints for the original CUSTOMERS table?
That is correct. When we use CREATE TABLE ... AS SELECT the statement creates a new table with the projection, column names and datatypes of the original tables (assuming a vanilla SELECT clause) and the data (determined by the WHERE clause, if any). However, the only constraints which are created are NOT NULL constraints on the primary key column(s) and any other mandatory columns. The new table does not have primary key, foreign key or check constraints. We have to create these explicitly.
Hence, this query ...
SELECT * FROM USER_CONSTRAINTS
WHERE TABLE_NAME = 'TEMP_CUST';
... might return fewer constraints than you were expecting.
Related
I am new to Hive and have some problems. I try to find a answer here and other sites but with no luck... I also tried many different querys that come to my mind, also without success.
I have my source table and i want to create new table like this.
Were:
id would be number of distinct counties as auto increment numbers and primary key
counties as distinct names of counties (from source table)
You could follow this approach.
A CTAS(Create Table As Select)
with your example this CTAS could work
CREATE TABLE t_county
ROW FORMAT DELIMITED FIELDS TERMINATED BY ','
STORED AS TEXTFILE AS
WITH t AS(
SELECT DISTINCT county, ROW_NUMBER() OVER() AS id
FROM counties)
SELECT id, county
FROM t;
You cannot have primary key or foreign keys on Hive as you have primary key on RBDMSs like Oracle or MySql because Hive is schema on read instead of schema on write like Oracle so you cannot implement constraints of any kind on Hive.
I can not give you the exact answer because of it suppose to you must try to do it by yourself and then if you have a problem or a doubt come here and tell us. But, what i can tell you is that you can use the insertstatement to create a new table using data from another table, I.E:
create table CARS (name string);
insert table CARS select x, y from TABLE_2;
You can also use the overwrite statement if you desire to delete all the existing data that you have inside that table (CARS).
So, the operation will be
CREATE TABLE ==> INSERT OPERATION (OVERWRITE?) + QUERY OPERATION
Hive is not an RDBMS database, so there is no concept of primary key or foreign key.
But you can add auto increment column in Hive. Please try as:
Create table new_table as
select reflect("java.util.UUID", "randomUUID") id, countries from my_source_table;
Just want to know is this possible.
Say that if i have value 'X' and iam sure that this is referenced in some other tables as PK value but not sure about exactly which table is that, so i would like to know the list of those tables.
Pseudo query of above what i mentioned
SELECT TABLE_NAME FROM DBA_TABLES WHERE <<ATLEAST ONE OF THE TABLE ROW PK VALUE IS MATCHING EQUAL TO 'X'>>;
I created a table using select with a union, as follows:
create table tableC as
select column1, column2 from tableA
union all
select column1, column2 from tableB
The resulting table (tableC) has inherited none of the constraints from tableA or tableB. Why weren't the constraints copied to the new table?
Using select ... as ... to create a table never copies constraints. If you want the new table to inherit constraints from the original tables, you must create the new constraints manually.
As #Davek points out, not null constraints will get copied from a single table select ... as .... I imagine that's because they are both column attributes and constraints. However, once the column has more than one source, it is reasonable that Oracle would not try to apply that constraint.
In response to the follow-up question "would it be possible to give tableC the same constraints either from tableA or tableB, after a CTAs?":
Of course it's possible, but there's no single command to do it. You could write a procedure that used dynamic SQL to copy the constraints. However, unless you're looking to automate this behavior, it'll generally be easier to extract the DDL using an IDE and change the table name.
I have a database which has a NOT NULL constraint on a field, and I want to remove this constraint. The complicating factor is that this constraint has a system-defined name, and that constraint's name differs between the production server, integration server, and the various developer databases. Our current process is to check in change scripts, and an automated task executes the appropriate queries through sqlplus against the target database, so I'd prefer a solution that could just be sent straight into sqlplus.
On my own database, the SQL to drop this would be:
alter table MYTABLE drop constraint SYS_C0044566
I can see the constraint when I query the all_constraints view:
select * from all_constraints where table_name = 'MYTABLE'
but I am not sure how to work with the SEARCH_CONDITION's LONG data type or how best to dynamically delete the looked-up constraint even after I know its name.
So, how can I create a change script that can drop this constraint based on what it is, rather than what its name is?
EDIT:
#Allan's answer is a good one, but I am concerned (in my lack of Oracle expertise) that it may not be universally true that any constraint that might have a system-generated name will have associated with it a way to remove the constraint without having to know its name. Is it true that there will always be a way to avoid having to know a system-named constraint's name when logically dropping that constraint?
alter table MYTABLE modify (MYCOLUMN null);
In Oracle, not null constraints are created automatically when not null is specified for a column. Likewise, they are dropped automatically when the column is changed to allow nulls.
Clarifying the revised question: This solution only applies to constraints created for "not null" columns. If you specify "Primary Key" or a check constraint in the column definition without naming it, you'll end up with a system-generated name for the constraint (and the index, for the primary key). In those cases, you'd need to know the name to drop it. The best advice there is to avoid the scenario by making sure you specify a name for all constraints other than "not null". If you find yourself in the situation where you need to drop one of these constraints generically, you'll probably need to resort to PL/SQL and the data-definition tables.
Try:
alter table <your table> modify <column name> null;
Just remember, if the field you want to make nullable is part of a primary key, you can't.
Primary Keys cannot have null fields.
To discover any constraints used, use the code below:
-- Set the long data type for display purposes to 500000.
SET LONG 500000
-- Define a session scope variable.
VARIABLE output CLOB
-- Query the table definition through the <code>DBMS_METADATA</code> package.
SELECT dbms_metadata.get_ddl('TABLE','[Table Described]') INTO :output FROM dual;
This essentially shows a create statement for how the referenced table is made. By knowing how the table is created, you can see all of the table constraints.
Answer taken from Michael McLaughlin's blog: http://michaelmclaughlin.info/db1/lesson-5-querying-data/lab-5-querying-data/ From his Database Design I class.
I was facing the same problem trying to get around a custom check constraint that I needed to updated to allow different values. Problem is that ALL_CONSTRAINTS does't have a way to tell which column the constraint(s) are applied to. The way I managed to do it is by querying ALL_CONS_COLUMNS instead, then dropping each of the constraints by their name and recreate it.
select constraint_name
from all_cons_columns
where table_name = [TABLE_NAME]
and column_name = [COLUMN_NAME];
Something like that happened to me when I made copies of structures to temporary tables, so I removed the not null.
DECLARE
CURSOR cur_temp_not_null IS
SELECT table_name, constraint_name FROM all_constraints WHERE table_name LIKE 'TEMP_%' AND owner='myUSUARIO';
V_sql VARCHAR2(200);
BEGIN
FOR c_not_null IN cur_temp_not_null
LOOP
v_sql :='ALTER TABLE ' || c_not_null.table_name || ' DROP CONSTRAINT '|| c_not_null.constraint_name;
EXECUTE IMMEDIATE v_sql;
END LOOP;
END;
If constraint on column STATUS was created without a name during creating a table, Oracle will assign a random name for it. Unfortunately, we cannot modify the constraint directly.
Steps involved of dropping unnamed constraint linked to column STATUS
Duplicate STATUS field into a new field STATUS2
Define CHECK constraints on STATUS2
Migrate data from STATUS into STATUS2
Drop STATUS column
Rename STATUS2 to STATUS
ALTER TABLE MY_TABLE ADD STATUS2 NVARCHAR2(10) DEFAULT 'OPEN';
ALTER TABLE MY_TABLE ADD CONSTRAINT MY_TABLE_CHECK_STATUS CHECK (STATUS2 IN ('OPEN', 'CLOSED'));
UPDATE MY_TABLE SET STATUS2 = STATUS;
ALTER TABLE MY_TABLE DROP COLUMN STATUS;
ALTER TABLE MY_TABLE RENAME COLUMN STATUS2 TO STATUS;
I have this table in an Oracle DB which has a primary key defined on 3 of the data columns. I want to drop the primary key constraint to allow rows with duplicate data for those columns, and create a new column, 'id', to contain an auto-incrementing integer ID for these rows. I know how to create a sequence and trigger to add an auto-incrementing ID for new rows added to the table, but is it possible to write a PL/SQL statement to add unique IDs to all the rows that are already in the table?
Once you have created the sequence:
update mytable
set id = mysequence.nextval;
If you're just using an integer for a sequence you could update the id with the rownum. e.g.
update
table
set id = rownum
You then need to reset the sequence to the next valid id.
Is this what you need?
UPDATE your_table
SET id = your_seq.nextval;
This assumes you don't care what order your primary keys are in.
First you should check your PCTFREE... is there enough room for every row to get longer?
If you chose a very small PCTFREE or your data has lots of lenght-increasing updates, you might begin chaining every row to do this as an update.
You almost certainly better to do this as a CTAS.
Create table t2 as select seq.nextval, t1.* from t1.
drop t1
rename t2 to t1.