SQL AND vs. OR: How Does Operator Precedence Affect Query Results?

SQL logical operator precedence: subtle differences between AND and OR

In SQL, the logical operators AND and OR determine how to combine multiple conditions to filter data. Understanding their priorities is critical to ensuring that queries execute correctly and avoid unexpected results.

Understanding operator precedence

Priority defines the order in which operators in an expression are evaluated. In SQL, AND has higher priority than OR. This means AND operations will be performed before OR operations.

Compare two statements

Consider the following two SQL statements:

SELECT [...]
FROM [...]
WHERE some_col in (1,2,3,4,5) AND some_other_expr

SELECT [...]
FROM [...]
WHERE some_col in (1,2,3) or some_col in (4,5) AND some_other_expr

If you don’t understand operator precedence, you may think that these two statements are equivalent. However, they are not.

AND priority

In the second statement, the expression some_col in (1,2,3) or some_col in (4,5) is evaluated first because OR has lower precedence than AND. The result of this expression is a Boolean value indicating whether some_col is in any of the specified sets. Then, use the AND operator to combine this boolean value with some_other_expr.

Equivalent correct syntax

To make the second statement equivalent to the first, we need to group the two OR conditions using parentheses to override the precedence rules:

WHERE (some_col in (1,2,3) or some_col in (4,5)) AND some_other_expr

With this modification, the inner OR operation is evaluated first and then the resulting boolean is combined with some_other_expr using AND. The result is the same as the first statement.

Verify truth table

You can use a truth table to verify the difference between these two statements:

some_col	some_other_expr	语句1	语句2
1	true	true	true
2	true	true	true
3	true	true	true
4	true	true	false
5	true	true	false
1	false	false	false
2	false	false	false
3	false	false	false
4	false	false	false
5	false	false	false

As the table shows, the two statements produce different results for some_col values ​​that are not in the two sets specified in the OR condition.

Conclusion

Understanding SQL logical operator precedence is crucial to avoid unexpected query results. Remember that AND has higher precedence than OR, and use parentheses to group expressions as necessary to ensure the desired order of execution.

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