Differences

This shows you the differences between two versions of the page.

--- transformations:filter [2015/03/20 21:33] – elenap
+++ transformations:filter [2015/06/12 21:20] – dmitry
@@ Line 1: / Line 1: @@
 ===== Filter by Condition =====
-This transformation calculates an [[syntax:start|expression]] for every row (without adding a new column) and leaves only rows where this expression is true. Other rows are removed.
+This transformation calculates an [[syntax:start|expression]] for every row (without adding a new column) and keeps only rows where this expression is true. Other rows are removed.
-The expression should be logical and return a boolean value (i.e. either true or false).
+The expression must be a valid condition -- it must return either TRUE or FALSE for any row in the table. If it results in a non-boolean value (i.e. not TRUE or FALSE) for at least one row it will cause a project execution error.
 **Examples of conditions**
@@ Line 15: / Line 15: @@
-If no rows satisfy the condition then all rows get removed.
+If no rows satisfy the condition then all rows are removed.
+**Example of transformation**
+**Source table:** The longest rivers in the world
+^  River  ^  Length (km)  ^  Continent  ^
+| Nile  |  6650  |  Africa  |
+| Amazon  |  6400  |  South America  |
+| Yangtze  |  6300  |  Asia  |
+| Mississippi  |  6275  |  North America  |
+| Yenisei  |  5539  |  Asia  |
+| Yellow River  |  5464  |  Asia  |
+| Ob  |  5410  |  Asia  |
+| Paraná  |  4880  |  South America  |
+**Objective:** Find all rivers that are longer than 6000 km
+**Transformation parameters:**
+  * Column: Length (km)
+  * Expression: [Length (km)] > 6000
+**Output table:**
+^  River  ^  Length (km)  ^  Continent  ^
+| Nile  |  6650  |  Africa  |
+| Amazon  |  6400  |  South America  |
+| Yangtze  |  6300  |  Asia  |
+| Mississippi  |  6275  |  North America  |
+=== See also ===
+  * [[transformations:selectionfilter|Filter]]
+  * [[syntax:start|Expression syntax]]
+  * [[syntax:errorpropagation|Error propagation in expressions]]