Difference between decode and casewhen _ MySQL

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Difference between decode and case when

Sum: sum (decode (sign (xingji-6), '', h. real_count, 1, h. real_count, 0 ))

Compared with case when, decode cannot use and to connect multiple conditions in a function, but can only use function nesting.

Example: sum (decode (sign (xingji-6),-1, decode (hh. quxian, 18, h. real_count, 19, h. real_count, 20, h. real_count, 24, h. real_count,
34, h. real_count, 35, h. real_count, 0), 0 ))

Sum: sum (case when start_time = '24' then quantity else 0 end)

Case when is a little bloated, but it is more convenient to connect multiple conditions, for example: sum (case when
Start_time = '24' and city_code = '010' then quantity else 0 end)

The sum () function is used for Summation. the sign () function is used to return whether the return parameter is greater than 0, or less than 0 or equal to 0,
An exception is null :'';

For example, sign (1) ---> 1, sign (-1) --->-1, sign (0) ---> 0, sign (null) ---> '';

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