Constraints about - constraints

Question

What constraints are copied with table when we create table by using create table as select statement..?

Answer 1

I don't know about MySQL, but - as of Oracle - only the NOT NULL constraint. Even if there was a column designed as a primary key (which - implicitly - means that it can't be NULL), newly created table won't have that constraint.
For example:
SQL> create table test_1
2 (id number primary key,
3 name varchar2(20) not null);
Table created.
SQL> select constraint_name, constraint_Type from user_constraints where table_name = 'TEST_1';
CONSTRAINT_NAME C
-------------------- -
SYS_C009194 C --> "C"heck (NOT NULL) constraint
SYS_C009195 P --> "P"rimary key constraint
SQL> create table test_2 as select * from test_1;
Table created.
SQL> select constraint_name, constraint_Type from user_constraints where table_name = 'TEST_2';
CONSTRAINT_NAME C
-------------------- -
SYS_C009196 C --> that's the "C"heck - NOT NULL constraint
SQL> desc test_2
Name Null? Type
----------------------------------------- -------- ----------------------------
ID NUMBER
NAME NOT NULL VARCHAR2(20)
SQL>

Answer 2

MySQL would only preserve NOT NULL as constarint
CREATE TABLE tab1(id int AUTO_INCREMENT PRIMARY KEY)
create table tab2
(id int primary key,
col1 varchar(20) not null,
col2 int UNIQUE,
col3 int, FOREIGN KEY (col3) REFERENCES tab1(id))
CREATE TABLE tab3 AS SELECT * FROM tab2
Records: 0 Duplicates: 0 Warnings: 0
SHOW CREATE TABLE tab3
Table
Create Table
tab3
CREATE TABLE `tab3` (  `id` int NOT NULL,  `col1` varchar(20) NOT NULL,  `col2` int DEFAULT NULL,  `col3` int DEFAULT NULL) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci
fiddle