PostgreSQL Create Table: Create a table to allow any two of the column combination to contain unique values
12. Write a SQL statement to create a table countries, including country_id, country_name and region_id and make sure that the combination of columns country_id and region_id will be unique.
CREATE TABLE IF NOT EXISTS countries_n ( COUNTRY_ID varchar(2) NOT NULL UNIQUE DEFAULT '', COUNTRY_NAME varchar(40) DEFAULT NULL, REGION_ID decimal(10,0) NOT NULL, PRIMARY KEY (COUNTRY_ID,REGION_ID));
postgres=# CREATE TABLE IF NOT EXISTS countries ( postgres(# COUNTRY_ID varchar(2) NOT NULL UNIQUE DEFAULT '', postgres(# COUNTRY_NAME varchar(40) DEFAULT NULL, postgres(# REGION_ID decimal(10,0) NOT NULL, postgres(# PRIMARY KEY (COUNTRY_ID,REGION_ID)); CREATE TABLE
Here is the command to see the structure of the created table :
postgres=# \d countries Table "public.countries" Column | Type | Modifiers --------------+-----------------------+---------------------------------------- country_id | character varying(2) | not null default ''::character varying country_name | character varying(40) | default NULL::character varying region_id | numeric(10,0) | not null Indexes: "countries_pkey" PRIMARY KEY, btree (country_id, region_id) "countries_country_id_key" UNIQUE CONSTRAINT, btree (country_id)
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Previous: Write a SQL statement to create a table countries including columns country_id, country_name and region_id and make sure that the column country_id will be unique and store an auto-incremented value.
Next: Write a SQL statement to create a table job_history, including employee_id, start_date, end_date, job_id and department_id and make sure that, the employee_id column does not contain any duplicate values at the time of insertion and the foreign key column job_id contain only those values which exist in the jobs table.
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