In DolphinDB, how to set multiple query conditions as np.select() does?

Question

To set multiple query conditions, the CASE WHEN statement can be used in SQL in the case of a specified number of conditions; otherwise, the coalesce function can be used for dynamic SQL queries. I wonder how to perform multiple-condition queries with DolphinDB scripting language.

The following script is written in the Q language:

def^(^/)?'[conds;choices;nil]

I use metaprogramming to achieve multiple-condition queries in DolphinDB, as shown below:

unifedCall(coalesce, iif:E(conds. choices, nil).matrix2tuple()).nullFill(nil)

I wonder how to convert a matrix into a tuple or apply the coalesce function to a matrix with DolphinDB scripts.

Answer 1

To achieve multiple-condition queries using metaprogramming, you can refer to the following script:

unifiedCall(coalesce, loop(iif, conds, choices, NULL)).nullFill!(default)

conds is a list of multiple conditions and choices is a list of arrays from which the elements are selected based on the corresponding true conditions in conds. The loop template always returns a tuple. The input parameter args of the unifiedCall function is a tuple. You can use this feature to generate metacode.