Second Largest Salary

Payroll analytics pipelines commonly query compensation tables to set salary band thresholds and flag outlier pay structures. A typical step in such pipelines is identifying the second-highest distinct salary — used by HR systems to define the upper boundary of the second salary band and to detect when a single outlier is distorting the entire pay distribution. Your task is to find the second largest distinct salary from the `employees` table. If there is no second largest salary (only one distinct salary value exists, or the table is empty), return `NULL`.