query_script_if_else
QueryScript Flow Control: if-else statement
SYNOPSIS
if (expression) statement; [else statement;]
DESCRIPTION
if-else is a flow control branching structure. It makes for a condition test based on a given expression.
When the expression holds true, the statement (or block of statements) following the if statement are executed. The else clause is optional, and it's statement(s) are executed when the expression does not hold true.
There is no built-in elseif clause. However, a chained if-else if-else statement is valid.
Empty statements are not allowed in QueryScript. However, empty blocks are, and a if or else clause may use an empty block statement, or by the do-nothing pass statement.
EXAMPLES
Simple if-else condition:
set @x := 17; if (@x mod 2 = 0) SELECT 'even' AS answer; else SELECT 'odd' AS answer;
DELETE statement as expression:
set @country := 'USA';
if (DELETE FROM world.Country WHERE Code = @country)
{
-- We don't have foreign keys on these tables; do a manual drill down:
DELETE FROM world.City WHERE CountryCode = @country;
DELETE FROM world.CountryLanguage WHERE CountryCode = @country;
}
Using if to break out of a while loop:
CREATE TEMPORARY TABLE test.numbers (n INT UNSIGNED NOT NULL PRIMARY KEY);
INSERT INTO test.numbers (n) VALUES (17);
set @n := 0;
while (@n < 20)
{
if (INSERT IGNORE INTO test.numbers (n) VALUES (@n))
{
}
else
{
break;
}
set @n := @n + 1;
}
SELECT @n AS inserted_up_to_this_value;
+---------------------------+
| inserted_up_to_this_value |
+---------------------------+
| 17 |
+---------------------------+