The CUSTOMERS table has these columns: CUSTOMER_ID NUMBER(4) NOT NULL CUSTOMER_NAME VARCHAR2(100) NOT NULL STREET_ADDRESS VARCHAR2(150) CITY_ADDRESS VARCHAR2(50) STATE_ADDRESS VARCHAR2(50) PROVINCE_ADDRESS VARCHAR2(50) COUNTRY_ADDRESS VARCHAR2(50) POSTAL_CODE VARCHAR2(12) CUSTOMER_PHONE VARCHAR2(20) The CUSTOMER_ID column is the primary key for the table. You need to determine how dispersed your customer base is. Which expression finds the number of different countries represented in the CUSTOMERS table?()
评估以下CREATETABLE语句的执行结果: CREATET ABLE customers (customer_id NUMBER,customer_name VARCHAR2(25), address VARCHAR 2(25), city VARCHAR 2(25), region VARCHAR 2(25), postal_code VARCHAR 2(11), CONSTRAINT customer_id_un UNIQUE(customer_id), CONSTRAINTcustomer_name_nnNOTNULL(customer_name)); 为什么执行时此语句会失败()