Subquery

[Rule] Do not use repeated subquery statements in an SQL statement.
[Recommendation] Do not use scalar subqueries.
A scalar subquery is a subquery whose result is one value and whose condition expression uses an equal operator. Example:

The following statement does not meet specifications:
```
SELECT * FROM t1 WHERE id = (SELECT id FROM t2 LIMIT 1);
```
You are advised to split the preceding statement into two SQL statements and execute the subquery first.
[Recommendation] Do not use subqueries in the SELECT target columns. Otherwise, the plan cannot be pushed down, affecting the execution performance.
[Recommendation] The nesting depth of a subquery cannot exceed 2.
Subqueries cause temporary table overhead. Therefore, complex queries must be optimized based on service logic.