MYSQL 自定义函数（if else）

IF OBJECT_ID('dbo.GetDateDiff', 'FN') IS NOT NULL OR OBJECT_ID('dbo.GetDateDiff', 'TF') IS NOT NULL OR OBJECT_ID('dbo.GetDateDiff', 'IF') IS NOT NULL DROP FUNCTION dbo.GetDateDiff     //判断此函数是否存在，若存在，

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