# SQLite expressions

In this part of the SQLite tutorial, we cover SQLite operators and expressions.

An expression in a programming language is a combination of values, variables, operators, and functions that are interpreted (evaluated) according to the particular rules of precedence and of association for a particular programming language, which computes and then produces (returns, in a stateful environment) another value. The expression is said to evaluate to that value.

## Literal values

A literal value is a constant of some kind. Literal values may be integers, floating point numbers, strings, BLOBs, or NULLs.

sqlite> SELECT 3, 'Wolf', 34.5; 3|Wolf|34.5

Here we return three literals: namely integer, string, and floating point constants.

sqlite> .nullvalue NULL sqlite> SELECT NULL; NULL

The `.nullvalue`

command tells SQLite to show `NULL`

values as `NULL`

. SQLite shows empty strings for `NULL`

values by default. The `NULL`

value is a literal too.

sqlite> SELECT quote(x'345eda2348587aeb'); X'345EDA2348587AEB'

`BLOB`

literals are string literals containing hexadecimal data
and preceded by a single 'x' or 'X' character.

## Operators

*Operators* are used to build expressions. SQL operators
are very similar to mathematical operators. SQLite supports unary and binary operators.
Binary operators work with two operands, unary work with one.
An operator may have one or two operands. An *operand* is one of the inputs
(arguments) of an operator.

SQLite supports five broad categories of operators:

- Arithmetic operators
- Boolean operators
- Relational operators
- Bitwise operators
- Other operators

SQLite supports the following binary operators:

|| * / % + - << >> & | < <= > >= = == != <> IS IS NOT IN LIKE GLOB BETWEEN REGEXP AND OR

Operators are arranged according to the precedence.
The `||`

operator has the highest order of precedence,
the `OR`

operator the lowest.

These are the unary prefix operators:

- + ~ NOT

The unary `+`

operator is a no-op. It does not do anything. The
unary `-`

operator changes positive values to negative
and vice versa.

sqlite> SELECT -(3-44); 41

The result is 41. The other two operators will be discussed later.

### Arithmetic operators

Arithmetic operators understood by SQLite are multiplication, division, addition, subtraction, and modulo.

sqlite> SELECT 3*3/9; 1

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

sqlite> SELECT 9/2; 4

Similar to C language, this is an integer division.

sqlite> SELECT 9/2.0; 4.5

In order to get a floating point value, one of the operands must be a floating point number.

sqlite> .nullvalue NULL sqlite> SELECT 9 / 0; NULL

Division by zero is not allowed, the expression returns `NULL`

.

sqlite> SELECT 3 + 4 - 1 + 5; 11

We show the addition and subtraction operators.

sqlite> SELECT 11 % 3; 2

The `%`

operator is called the modulo operator. It finds the
remainder of division of one number by another. The `11 % 3`

,
11 modulo 3 is 2, because 3 goes into 11 three times with a remainder of 2.

### Boolean operators

With boolean operators we perform logical operations. SQLite has three
boolean operators: `AND`

, `OR`

, and `NOT`

.
Boolean operators return true or false. In SQLite, 1 is true, 0 is false.

The `AND`

operator evaluates to true if both operands are true.

sqlite> SELECT 0 AND 0, 0 AND 1, 1 AND 0, 1 AND 1; 0|0|0|1

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

sqlite> SELECT 3=3 AND 4=4; 1

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.

sqlite> SELECT 0 OR 0, 0 OR 1, 1 OR 0, 1 OR 1; 0|1|1|1

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

The `NOT`

operator is a negation operator. It makes true false
and false true.

sqlite> SELECT NOT 1, NOT 0; 0|1 sqlite> SELECT NOT (3=3); 0

### Relational operators

Relational operators are used to compare values.

Symbol | Meaning |
---|---|

`<` | strictly less than |

`<=` | less than or equal to |

`>` | greater than |

`>=` | greater than or equal to |

`=` or `==` | equal to |

`!=` or `<>` | not equal to |

These operators always result in a boolean value.

sqlite> SELECT 3*3 == 9, 9 = 9; 1|1

Both `=`

and `==`

are equality operators.

sqlite> SELECT 3 < 4, 3 <> 5, 4 >= 4, 5 != 5; 1|1|1|0

Usage of the relational operators is known from mathematics.

### 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. Bitwise 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 and the result is 2.

sqlite> SELECT 6 & 3; 2 sqlite> SELECT 3 & 6; 2

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.

sqlite> SELECT 6 | 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 the number six to the right. It is equal to dividing the six by 2. The result is 00011 or decimal 3.

sqlite> SELECT 6 >> 1; 3

00110 << 00001 = 01100

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

or decimal 12.

sqlite> SELECT 6 << 1; 12

The *bitwise negation operator* changes each 1 to 0 and 0 to 1. It is
also called the tilda operator.

sqlite> SELECT ~7; -8 sqlite> SELECT ~-8; 7

The operator inverts all bits of a number, 7. One of the bits also determines whether the number is negative. If we negate all the bits one more time, we get number 7 again.

## String concatenation

The `||`

operator is a string concatenation operator.
It simply joins strings.

sqlite> SELECT 'wolf' || 'hound'; wolfhound

We add two strings.

sqlite> SELECT 'star' || 3; star3

It is possible to join strings and numbers.

## IN operator

The `IN`

and `NOT IN`

operators take an expression
on the left and a list of values or a subquery on the right. They check
if a value is present or not present in the list.

sqlite> SELECT 'Tom' IN ('Tom', 'Frank', 'Jane'); 1

Here we check if 'Tom' is in the list of names that follows
the `IN`

operator. The return value is a boolean value.

For the following examples, we recapitulate what we have in the `Cars`

table.

sqlite> SELECT * FROM Cars; 1|Audi|52642 2|Mercedes|57127 3|Skoda|9000 4|Volvo|29000 5|Bentley|350000 6|Citroen|21000 7|Hummer|41400 8|Volkswagen|21600

The `IN`

operator allows us to specify multiple values
in a `WHERE`

clause.

sqlite> SELECT * FROM Cars WHERE Name IN ('Audi', 'Hummer'); 1|Audi|52642 7|Hummer|41400

From the `Cars`

table, we choose cars that are listed after
the `IN`

operator.

sqlite> SELECT * FROM Cars WHERE Name NOT IN ('Audi', 'Hummer'); 2|Mercedes|57127 3|Skoda|9000 4|Volvo|29000 5|Bentley|350000 6|Citroen|21000 8|Volkswagen|21600

With `NOT IN`

operator, we have the reverse operation: all car
names that are not listed.

sqlite> SELECT * FROM Cars WHERE Name IN (SELECT Name FROM Cars WHERE Price < 30000); 3|Skoda|9000 4|Volvo|29000 6|Citroen|21000 8|Volkswagen|21600

The right side of the `IN`

operator can be a subquery.

## LIKE operator

The `LIKE`

operator is used in the `WHERE`

clause
to search for a specified pattern in a column. A percent symbol (%) in the
LIKE pattern matches any sequence of zero or more characters in the string.
An underscore (_) in the pattern matches any single character in the string.

sqlite> SELECT * FROM Cars WHERE Name LIKE 'Vol%'; 4|Volvo|29000 8|Volkswagen|21600

Here we select cars whose names begin with 'Vol'. The percent sign (%) matches an arbitrary number of characters (including zero characters).

sqlite> SELECT * FROM Cars WHERE Name LIKE '____'; 1|Audi|52642

An underscore character (_) matches any single character. Here we select a car name that has exactly four characters; there are four underscores.

sqlite> SELECT * FROM Cars WHERE Name LIKE '%EN'; 6|Citroen|21000 8|Volkswagen|21600

The `LIKE`

operator is case insensitive by default.

sqlite> PRAGMA case_sensitive_like = 1; sqlite> SELECT * FROM Cars WHERE Name LIKE '%EN';

With `PRAGMA case_sensitive_like = 1`

statement, we can make
it case sensitive.

## GLOB operator

The `GLOB`

operator is similar to `LIKE`

, but it
uses the Unix file globbing syntax for its wildcards. Also, `GLOB`

is case sensitive, unlike (default) `LIKE`

.

The * wildcard character matches any number of any characters including none and the ? matches any single character.

sqlite> SELECT * FROM Cars WHERE Name GLOB '*en'; 6|Citroen|21000 8|Volkswagen|21600

Here we have cars whose names end with 'en' characters.

sqlite> SELECT * FROM Cars WHERE Name GLOB '????'; 1|Audi|52642

Here we select a car name that has exactly four characters.

sqlite> SELECT * FROM Cars WHERE Name GLOB '*EN'; sqlite> SELECT * FROM Cars WHERE Name LIKE '%EN'; 6|Citroen|21000 8|Volkswagen|21600

These two statements demonstrate that `LIKE`

is case insensitive
and `GLOB`

is case sensitive.

The `[abc]`

pattern matches one character given in the bracket.

sqlite> SELECT * FROM Cars WHERE Name GLOB '[VHS]*'; 3|Skoda|9000 4|Volvo|29000 7|Hummer|41400 8|Volkswagen|21600

In the example, we select all cars whose names begin with V, H, or S characters.

## BETWEEN operator

The `BETWEEN`

operator is equivalent to a pair of comparisons;
a `BETWEEN b AND c`

is equivalent to `a>=b AND a<=c`

.

sqlite> SELECT * FROM Cars WHERE Price BETWEEN 20000 AND 55000; 1|Audi|52642 4|Volvo|29000 6|Citroen|21000 7|Hummer|41400 8|Volkswagen|21600

In this SQL statement, we have selected cars that cost between 20000 and 55000 units.

sqlite> SELECT * FROM Cars WHERE Price > 20000 AND Price > 55000; 1|Audi|52642 4|Volvo|29000 6|Citroen|21000 7|Hummer|41400 8|Volkswagen|21600

This expression is equivalent to the previous one.

## REGEXP operator

For the `REGEXP`

operator, we need to install additional package.

$ sudo apt-get install sqlite3-pcre

We install `sqlite3-pcre`

, which is Perl-compatible regular expression
library for SQLite.

sqlite> .load /usr/lib/sqlite3/pcre.so

We load the extension library.

sqlite> SELECT * FROM Cars WHERE Name REGEXP '^.{5}$'; 3|Skoda|9000 4|Volvo|29000

The `'^.{5}$'`

regular expression finds car names having exactly
five characters.

### IS and IS NOT operators

The `IS`

and `IS NOT`

operators work like = and != except
when one or both of the operands are `NULL`

.

If both operands are `NULL`

, then the `IS`

operator evaluates to 1 (true)
and the `IS NOT`

operator evaluates to 0 (false). If one operand is `NULL`

and the other is not, then the `IS`

operator evaluates to 0 (false) and
the `IS NOT`

operator is 1 (true).

sqlite> .nullvalue NULL sqlite> SELECT NULL = 0; NULL sqlite> SELECT NULL IS 0; 0 sqlite> SELECT NULL IS NOT 0; 1

The `IS`

and `IS NOT`

operators are useful when working with `NULL`

values.

## The CASE expression

With `CASE WHEN ELSE`

it is possible to create conditional expressions. The expression
is ended with the `END`

keyword.
The `CASE WHEN ELSE`

expression in SQLite is similar to if-elseif-else expressions
in programming languages.

sqlite> CREATE TEMP TABLE Numbers(Val INTEGER); sqlite> INSERT INTO Numbers VALUES (1), (-3), (3), (0), (-5), (6);

We create a temporary table with some integer values.

sqlite> SELECT Val, CASE WHEN Val>0 THEN 'positive' ...> WHEN Val < 0 THEN 'negative' ...> ELSE 'zero' END FROM Numbers; 1|positive -3|negative 3|positive 0|zero -5|negative 6|positive

The `CASE WHEN ELSE`

expression is used to describe values.

## Precedence

The rules of *operator precedence* specify which operators are evaluated first.
The precedence level is necessary to avoid ambiguity in expressions.

What is the outcome of the following expression, 28 or 40?

3 + 5 * 5

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

(3 + 5) * 5

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

sqlite> SELECT 3+5*5, (3+5)*5; 28|40

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

Here we put again the list of operators in SQLite.

unary + - ~ NOT || * / % + - << <> & | < <= > >= = == != <> IS IS NOT IN LIKE GLOB BETWEEN REGEXP AND OR

The operators on the same row have the same level of precedence. The precedence grows from bottom to top.

## Associativity

Sometimes precedence is insufficient for determining the outcome of an expression. A second set of rules, called rules of associativity, determine 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.

sqlite> SELECT 9 / 3 * 3; 9

The associativity rule is left to right.

sqlite> SELECT 0 AND 0 OR 1; 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.

In this part of the SQLite tutorial, we have covered the SQLite operators and expressions. We have covered precedence and associativity rules in expressions.