In this part of the SQLite tutorial, we will cover SQLite expressions.
Wikipedia defines an expression in a programming language as 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 the SQLite to show NULL values as NULL. SQLite shows empty strings for NULL values by default. NULL value is a literal too.
sqlite> SELECT 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. There are two kinds of operators. Binary and unary. 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.
There have several types of operators:
- Arithmetic operators
- Boolean operators
- Relational operators
- Bitwise operators
- Other operators
SQLite understand the following binary operators.
|| * / % + - << <> & | < <= > >= = == != <> IS IN LIKE GLOB BETWEEN 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 + 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 on.
Arithmetic operators
Arithmetic operators understood by SQLite are: multiplication, division, addition, subtraction and modulo.
sqlite> SELECT 3*3/9; 1
These are multiplication and division operators, that we know from mathematics.
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. 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 the following 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 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 comptuters. 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 nubmers. 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.
sqlite> SELECT 6 & 3; 2 sqlite> SELECT 3 & 6; 2
The bitwise or operator performs bit-by-bit comparison between two nubmers. 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 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 number six to the left. It is equal to muliplying 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 reverts all bits of a number 7. One of the bits also determines, whether the number is negative or not. If we negate all the bits one more time, we get number 7 again.
Other operators
There are some other operators left. These include ||, IN, LIKE, GLOB, BETWEEN.
sqlite> SELECT 'wolf' || 'hound'; wolfhound
The || operator is a string concatenation operator. It simply joins strings.
We can use the IN operator in two cases.
sqlite> SELECT 'Tom' IN ('Tom', 'Frank', 'Jane');
1
Here we check, if the string value 'Tom' is in the list of names, following the IN operator. The return value is a boolean value.
For the following example, let's recap, what we have in the Cars table.
sqlite> SELECT * FROM Cars; Id Name Cost ---------- ---------- ---------- 1 Audi 52642 2 Mercedes 57127 3 Skoda 9000 4 Volvo 29000 5 Bentley 350000 6 Citroen 21000 7 Hummer 41400 8 Volkswagen 21600
In the second case, the IN operator allows you to specify multiple values in a WHERE clause.
sqlite> SELECT * FROM Cars WHERE Name IN ('Audi', 'Hummer');
Id Name Cost
---------- ---------- ----------
1 Audi 52642
7 Hummer 41400
From the Cars table, we choose cars, that are listed after the IN operator.
The LIKE operator is used in a WHERE clause to search for a specified pattern in a column.
sqlite> SELECT * FROM Cars WHERE Name LIKE 'Vol%'; Id Name Cost ---------- ---------- ---------- 4 Volvo 29000 8 Volkswagen 21600
Here we select cars, whose names begin with 'Vol'.
sqlite> SELECT * FROM Cars WHERE Name LIKE '____'; Id Name Cost ---------- ---------- ---------- 1 Audi 52642
Here we select a car name, that has exactly four characters. There are four underscores.
The GLOB operator is similar to the LIKE, but uses the Unix file globbing syntax for its wildcards. Also, GLOB is case sensitive, unlike LIKE.
sqlite> SELECT * FROM Cars WHERE Name GLOB '*en'; Id Name Cost ---------- ---------- ---------- 6 Citroen 21000 8 Volkswagen 21600
Here we have cars, whose names end with 'en' characters.
sqlite> SELECT * FROM Cars WHERE Name GLOB '????'; Id Name Cost ---------- ---------- ---------- 1 Audi 52642
Here we again 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'; Id Name Cost ---------- ---------- ---------- 6 Citroen 21000 8 Volkswagen 21600
These two statements demonstrate, that the LIKE operator is case insensitive, GLOB is case sensitive.
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 Cost BETWEEN 20000 AND 55000; Id Name Cost ---------- ---------- ---------- 1 Audi 52642 4 Volvo 29000 6 Citroen 21000 7 Hummer 41400 8 Volkswagen 21600
In this SQL statement, we have selected cars, which cost between 20000 and 55000 units.
Precedence
The operator precedence tells us 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 square brackets. Expressions inside square brackets are always evaluated first.
sqlite> SELECT 3+5*5, (3+5)*5; 28|40
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.
Here we put again the list of operators in SQLite.
unary + - ~ NOT || * / % + - << <> & | < <= > >= = == != <> IS IN LIKE GLOB BETWEEN AND OR
The operators on the same row have the same level of precedence. The precedence grows from bottom to top.
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.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 expressions.
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