The action symbol that appears in the SQL Base statement-select statement

2. Action symbols that appear in the SELECT statement

2.1 Merging Operators
Select A.ename| | ' ' | | To_char (SAL) from EMP A;

2.2 To eliminate duplicate rows

SELECT DISTINCT deptno from EMP;

2.3 The difference between a space, an empty string, and a null

Select ASCII ('), ASCII (NULL), ASCII (') from dual;

Difference:
Explicitly, an empty string is the same value stored in the database as NULL, but NULL can be assigned to any data type, whereas an empty string can only be assigned to a string type

To filter null values, you need to use the following notation

SELECT * FROM EMP where comm are NOT null;

Typical errors (but no error)

SELECT * FROM emp where comm! = NULL;
SELECT * from emp where comm <> null;
SELECT * FROM emp where comm = null;

2.4 Comparison operations
= >= <= <>! = > < in like are NULL is not null between and not between and

SELECT * from EMP where Sal between and 1500; --Includes 800 and 1500
=
SELECT * from emp where Sal >= and sal<= 1500;

SELECT * from emp where Sal is not between and 1500;
=
SELECT * from emp where Sal < n or sal > 1500;

SELECT * from EMP where empno in (7499,7698,7788);
=
SELECT * from emp where empno=7499 or empno=7698 or empno=7788;

SELECT * from emp where ename like ' t% ';
SELECT * from emp where ename ' t% ';

SELECT * from emp where ename like '%t% ';
SELECT * from emp where ename '%t% ';

SELECT * from emp where sal! = 1000
=
SELECT * from emp where Sal <> 1000

2.5 Logical operators
Boolean operation (Boolean) evaluates to: TRUE, FALSE

Non-OR and

And

True and false = False
True and True = True
Flase and false = False


Or

True or false = True
True or True = True
Flase or false = False


Non -

Not true = False

Not false = True

SELECT * from emp where sal! = job= ' clerk '
SELECT * from emp where sal! = or job= ' clerk '

Attention:

Order of precedence issues:
Not > and > OR If you want to override the default priority order, you have to use parentheses

SELECT * from emp where not (job= ' Saleman ' or job= ' clerk ') and Sal > 1000

SELECT * from emp where not job= ' Saleman ' or job= ' clerk ' and sal > 1000

The action symbol that appears in the SQL Base statement-select statement