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lionel

519 post(s)
#30-Nov-18 22:17

In fehily book i can read than "Nevertheless , you won't be a competent SQL programmer without understanding null completely "

i search how mfd9 sql engine behave with null /insert return directly ( edit table) or after apply sql function or script that use table driver

if i read doc of GeomArea([Geom], 0) it return a <value> <NULL> for point and line and 0 for collapse area or value >0 for no collapse area .

Is there general documentation about null or because there is so much contet that better is foreach context study by ourown the behaviour of null ?

here some content refer to link find in manifold 9 documentation about null

null 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59

60 61 62 63 64 65 66 67 68 69 70 71 72

nullable

nulled

1 For example, if all of the fields in a table have NULL values in at least some records none of the fields will be suitable for use as an index. We will have to add a new field to the table and then create an index on it.

2 Aggregates ignore NULL values. First and Last return the first and last value respectively. Unlike most other aggregates, First and Last do not skip NULLs.

3 Renamed transforms: Enclosing Rect, Rotation Allowed -> Enclosing Rect with Rotation, Is NULL -> Null Values, Is not NULL -> Non-null Values. Renamed transform parameters: DX -> Delta X, DY -> Delta Y, Pattern (when used by Like) -> Pattern (%_).

> ? COALESCE(NULL

 NULL, 1/0, NULL, 5, 8)

float64: 5

The COALESCE function walks through the arguments and returns the first non-NULL value. The first four values are NULLs and are skipped. The fifth argument produces the result, 5, while the sixth argument is ignored.

5 the computed field that references a nonexistent countries table marks itself as having failed to load and returns NULL values.

6

Manifold Future - Future Tour Part 6 Cell Context Menu - A short video showing a fast and easy way to copy between cells in tables using the context menu. Also... one step undo of pending changes, setting the value of a cell to NULL and more. T

7 Delete - Delete the value in the cell, resulting in <NULL>

8 The Non-null Values and Null Values templates open only the Value combo box while other templates will open additional combo boxes to specify other parameters the template requires.

9=6

10 Filter

Enter text to be matched, case not significant. The list of operators and functions will be reduced to show only those items which match the text. For example, entering nullreduces the list to only those items that have NULL in their names.

11=6

12 Three new records appear in the table. Since we did not paste anything into the classfield, that field has NULL for each record.

13

The system will scan all values in the field to make sure there are no duplicates or NULLs

14 DISTINCT

NULLs - All NULL values are treated as equal to each other (the behavior prescribed by the standard and implemented in other databases). DISTINCT is one of those places where the notion of a NULL as a generic unknown value meets established practice: instead of treating all NULL values as different from all other values, including other NULL values, DISTINCT treats them as the same value. For example, using DISTINCT on a table with a single field will combine all NULL values into a single record.

15

To manually set the value of a cell to <NULL>, right-click onto the cell and choose Delete.

By default the New Field dialog lets us specify the name and the data type of the new field. We should, of course, set the field type to something sensible for the field we intend to add. For example, if want to add a field with names of people and we accidentally choose a type of tile instead of the nearby nvarchar when we try to populate the field with text the result will be NULLs.

Delete

Delete the contents of the cell, which creates a <NULL> value in the cell.

Delete - Delete the value in the cell, resulting in <NULL>

=6

16 The GeomArea function computes the area of area objects and returns NULL for line and point objects (which do not have dimensional area).

.....

regard's

Attachments:
null_test_v9.map


join image "Because my dad promised me" interstellar from Manifold: Time by Stephen Baxter. power Math destruction

adamw


8,259 post(s)
#12-Dec-18 09:06

Is there general documentation about null or because there is so much contet that better is foreach context study by ourown the behaviour of null ?

The general info about NULLs is that a NULL denotes an unknown value. Everything else flows from that.

However, that's just a general rule. In some cases, NULLs are handled slightly differently, for historical or practical reasons.

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