Variables & Operators in MySQL

Variables 

A variable is a characteristic notation with a Character value. This value may be changed over time.  Variables in MySQL are represented by the @ character.

mysql> SET @name = 'Arjun';

mysql> SELECT @name;

Operators :

Operators may be used to build expressions. operators are very similar to mathematical operators.  There are two kinds of operators. Binary and unary. Binary operators work with two operands,but unary work with one.

• An operator may have one or two operands.
• An operand is one of the inputs (arguments) of an operator.

There are different types of operators:

• Arithmetic operators
• Boolean operators
• Relational operators
• Bitwise operators
• Other operators
• Unary operators

Examples : unary operators.

mysql> SELECT +5, 9;
The + is a no-op. It does not do anything.

mysql> SELECT -(4-54);
The - unary operator changes positive values to negative and vice versa.

mysql> SELECT NOT (4>8);

The NOT operator negates a value. The result of the4>8 comparison is false and the negation operator negates it to true.

Arithmetic operators

These are some arithmetic operators : multiplication, division, integer division, addition, subtraction and modulo.

mysql> SELECT 4 + 7 - 1;

Addition and subtraction operators.

mysql> SELECT 4*3/6;

These are multiplication and division operators, that we know from mathematics.

mysql> SELECT 7/3, 7 DIV 3;

The above SQL statement shows the difference between the division and integer division operators. The first returns a floating point number, the second returns an integer.

mysql> SELECT 15 %43;

The % operator is called the modulo operator. It finds the remainder of division of one number by another. 11 % 3, 11 modulo 3 is 2, because 3 goes into 11 three times with a remainder of 2.

Logical operators

MySQL understands these logical operators: AND, OR and NOT and XOR. Logical operators return TRUE or FALSE. In MySQL, 1 is true, 0 is false.

The AND operator evaluates to true, if both operands are true.

mysql> SELECT FALSE AND FALSE, FALSE AND TRUE,
        TRUE AND FALSE, TRUE AND TRUE;

The first three operations evaluate to false, the last one to true.

mysql> SELECT 5=5 AND 6=6;

Both operands are true, so the result is true (1).

The OR operator evaluates to true, if at least one of the operands is true.

mysql> SELECT FALSE OR FALSE, FALSE OR TRUE,
          TRUE OR FALSE, TRUE OR TRUE;

The first operation evaluates to false, other operations evaluate to true.

The XOR operator evaluates to true, if exactly one of the operands is true.

mysql> SELECT FALSE XOR FALSE, FALSE XOR TRUE,
          TRUE XOR FALSE, TRUE XOR TRUE;

Two of the operations result in true.

The NOT operator is negation operator. It makes true false and false true.

mysql> SELECT NOT TRUE, NOT FALSE;
mysql> SELECT NOT (3=3);

 

Relational operators

Relational operators are used to compare values. These operators always result in boolean value.

mysql> SELECT 3*3=9, 9=9;

The = is the equality operator.

mysql> SELECT 3 < 4, 3 <> 5, 4 <= 4, 5 != 5;

Usage of the relational operators is known from mathematics.

Bitwise operators

In the Bitwise operators Decimal numbers are natural to humans. Binary numbers are native to computers. Binary, octal, decimal or hexadecimal symbols are only notations of the same number. These operators work with bits of a binary number. We have binary logical operators and shift operators.

The bitwise and operator performs bit-by-bit comparison between two numbers. The result for a bit position is 1 only if both corresponding bits in the operands are 1.    
    00110
  & 00011
  = 00010

The first number is a binary notation of 6. The second is 3. The result is 2.

mysql> SELECT 6 & 3, 3 & 6;

The bitwise or operator performs bit-by-bit comparison between two numbers. The result for a bit position is 1 if either of the corresponding bits in the operands is 1.

   
     00110
  |  00011
   = 00111
 

The result is 00110 or decimal 7.

mysql> SELECT 7 | 3, 3 | 7;

The bitwise shift operators shift bits to the right or left.

number << n : multiply number 2 to the nth power
number >> n : divide number by 2 to the nth power
 

These operators are also called arithmetic shift.

     00110
 >>  00001
   = 00011
  We shift each of the bits of number six to the right. It is equal to dividing the six by 2. The result is 00011 or decimal 3.

mysql> SELECT 6 >> 1;
      00110
  << 00001
   = 01100

We shift each of the bits of number six to the left. It is equal to multiplying the number six by 2. The result is 01100 or decimal 12.

mysql> SELECT 6 << 1;

Other operators

There are some other operators left. These include IS, IN, LIKE, REGEXP, BETWEEN.

The IS operator tests if an operand is a boolean value.

mysql> SET @running = FALSE;
mysql> SELECT @running IS FALSE;

 

We set a variable to boolean false. We check if the variable is FALSE using the IS operator.

We can use the IN operator in two cases.

mysql> SELECT 'Ram' IN ('Ram', 'Hari', 'Mohan');

Here we check, if the string value 'Ra,' is in the list of names, following the IN operator. The return is a boolean value.

Precedence

The operator precedence tells us which operators are evaluated first. The precedence level is necessary to avoid ambiguity in expressions.

4 + 5 * 5

Like in mathematics, the multiplication operator has a higher precedence than addition operator. So the outcome is 28.

(3 + 8) * 5

To change the order of evaluation, we can use square brackets. Expressions inside square brackets are always evaluated first.

mysql> SELECT 4+5*5, (3+8)*5;

The first expression evaluates to 28, because the multiplication operator has a higher precedence, than the addition one. In the second example, we have used square brackets to change the order of precedence. So the second expressions evaluates to 40.

Associativity

Sometimes the precedence is not satisfactory to determine the outcome of an expression. There is another rule called associativity. The associativity of operators determines the order of evaluation of operators with the same precedence level.

9 / 3 * 3

What is the outcome of this expression? 9 or 1? The multiplication, deletion and the modulo operator are left to right associated. So the expression is evaluated this way: (9 / 3) * 3 and the result is 9.

mysql> SELECT 9 / 3 * 3;

The associativity rule is left to right.

mysql> SELECT 0 AND 0 OR 1;

 

The associativity rule is again left to right. If it was right to left, the result would be 0.

Arithmetic, boolean, relational and bitwise operators are all left to right associated.