Using SQL, how do you find the child number in a table from two columns where they are parent-child-CodePudding

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Using SQL, how do you find the child number in a table from two columns where they are parent-child

Time：09-19

From this table, after querying I want to get the child column that shows the number of the child for each id.

I want:

CodePudding user response：

A scalar subquery will produce the data you want. For example:

select
  t.*,
  (select count(*) from t b where b.parent_id = t.id) as child
from t