SET
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SELECT
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Set statements are used only for assigning values to variables.
SET @x = 1;
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Select statements are used both for assigning values to variables and for querying data.
SELECT @x = 1;
SELECT @age = p.age
FROM persons p
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When performing assignments, SET can perform only one assignments at once.
SET @x = 1, @y = 2 would not work
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When performing assignments, SELECT can perform multiple assignments at once.
SELECT @x = 1, @y = 2 would work fine
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Performance wise, SET is a little bit slower when compared to SELECT. This is because the operator separately assigns values to each individual variable.
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SELECT is faster than SET in a way that it allows multiple assignments in a single go instead of making variable assignments separately.
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When making assignment from a query that returns more than one rows of result, SET will raise an error.
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When making assignment from a query that returns more than one rows of result, SELECT will assign the last result value to variable.
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When making assignment from a query that returns no results, SET will assign NULL to the variable
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When making assignment from a query that returns no results, SELECT will not make assignment so the variable will remain unchanged.
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