Generate nullable value from subselect statements

[ryu-ichiroh]

Hi maintainers.

Change subselect statements containing WHERE/HAVING clauses to be NULL.
Resolves: #1208

[ryu-ichiroh]

👀

[kyleconroy]

@ryu-ichiroh Could you please rebase and regenerate your test output with 1.19.1?

I am a bit concerned about this being a backwards incompatible change, but am leaning towards accepting this as subselects that can return NULL can't be used today without COALESCE checking.

[ryu-ichiroh]

Could you please rebase and regenerate your test output with 1.19.1?

Sure.

[ryu-ichiroh]

Could you please rebase and regenerate your test output with 1.19.1?

I've finished.
ce1b4c5

[ryu-ichiroh]

Let me have more time. I'll improve.

[ryu-ichiroh]

@kyleconroy
I've finished to fix all. Could you please review this again?

[ryu-ichiroh]

I've researched about cases a subselect returns null.

A column of no rows returns null. but if count() return 0.

• https://www.postgresql.org/docs/15/functions-aggregate.html
• https://dev.mysql.com/doc/refman/8.0/en/aggregate-functions.html
• https://www.sqlite.org/lang_aggfunc.html

According to above documentation, total() also returns 0 in sqlite.
Other aggregate functions returns null.

[ryu-ichiroh]

count() returns non-null.

[ryu-ichiroh]

When retrieving the first row, it returns null.

[ryu-ichiroh]

-- name: BarExists :one
SELECT
    EXISTS (
        SELECT
            1
        FROM
            bar
        where
            id = ?
    )

This is a regression. 😞
I understand that EXISTS doesn't return null. so It should be generated as non-null.
There are no tests of select_exists in MySQL, I think the same issue occur.
It doesn't occurred the issue in MySQL, because it is parsed ast.SubLink.
This is the issue specific to SQLite only.

I displayed the ast, but I couldn't find EXISTS clause.
Do you have any idea?

Expand to see AST.

&ast.SelectStmt{
  DistinctClause: (*ast.List)(nil),
  IntoClause:     (*ast.IntoClause)(nil),
  TargetList:     &ast.List{
    Items: []ast.Node{
      &ast.ResTarget{
        Name:        (*string)(nil),
        Indirection: (*ast.List)(nil),
        Val:         &ast.SelectStmt{
          DistinctClause: (*ast.List)(nil),
          IntoClause:     (*ast.IntoClause)(nil),
          TargetList:     &ast.List{
            Items: []ast.Node{
              &ast.ResTarget{
                Name:        (*string)(nil),
                Indirection: (*ast.List)(nil),
                Val:         &ast.A_Const{
                  Val: &ast.Integer{
                    Ival: 1,
                  },
                  Location: 134,
                },
                Location: 134,
              },
            },
          },
          FromClause: &ast.List{
            Items: []ast.Node{
              &ast.RangeVar{
                Catalogname:    (*string)(nil),
                Schemaname:     (*string)(nil),
                Relname:        &"bar",
                Inh:            false,
                Relpersistence: 0x00,
                Alias:          (*ast.Alias)(nil),
                Location:       161,
              },
            },
          },
          WhereClause: &ast.A_Expr{
            Kind: 0x0,
            Name: &ast.List{
              Items: []ast.Node{
                &ast.String{
                  Str: "=",
                },
              },
            },
            Lexpr: &ast.ColumnRef{
              Name:   "",
              Fields: &ast.List{
                Items: []ast.Node{
                  &ast.String{
                    Str: "id",
                  },
                },
              },
              Location: 191,
            },
            Rexpr: &ast.ParamRef{
              Number:   1,
              Location: 196,
              Dollar:   false,
            },
            Location: 0,
          },
          GroupClause: &ast.List{
            Items: []ast.Node{},
          },
          HavingClause: nil,
          WindowClause: &ast.List{
            Items: []ast.Node{},
          },
          ValuesLists: &ast.List{
            Items: []ast.Node{},
          },
          SortClause:    (*ast.List)(nil),
          LimitOffset:   nil,
          LimitCount:    nil,
          LockingClause: (*ast.List)(nil),
          WithClause:    &ast.WithClause{
            Ctes: &ast.List{
              Items: []ast.Node{},
            },
            Recursive: false,
            Location:  0,
          },
          Op:   0x0,
          All:  false,
          Larg: (*ast.SelectStmt)(nil),
          Rarg: (*ast.SelectStmt)(nil),
        },
        Location: 98,
      },
    },
  },
  FromClause: &ast.List{
    Items: []ast.Node{},
  },
  WhereClause: nil,
  GroupClause: &ast.List{
    Items: []ast.Node{},
  },
  HavingClause: nil,
  WindowClause: &ast.List{
    Items: []ast.Node{},
  },
  ValuesLists: &ast.List{
    Items: []ast.Node{},
  },
  SortClause:    (*ast.List)(nil),
  LimitOffset:   nil,
  LimitCount:    nil,
  LockingClause: (*ast.List)(nil),
  WithClause:    &ast.WithClause{
    Ctes: &ast.List{
      Items: []ast.Node{},
    },
    Recursive: false,
    Location:  0,
  },
  Op:   0x0,
  All:  false,
  Larg: (*ast.SelectStmt)(nil),
  Rarg: (*ast.SelectStmt)(nil),
}