Merge pull request #2920 from ClickHouse/add-kb-articles-lio

add kb articles on filtering using array-columns and ingest all rows

Author: gingerwizard

knowledgebase/how-to-filter-a-clickhouse-table-by-an-array-column.md

@@ -0,0 +1,102 @@
+---
+title: How to filter a ClickHouse table by an array-column?
+description: "Knowledgebase article on how to filter a ClickHouse table by an array-column."
+date: 2024-12-17
+---
+
+## Introduction
+
+Filtering a ClickHouse table by an array-column is a common task and the product offers a lot of [functions](/docs/en/sql-reference/functions/array-functions) to work with array-columns.
+
+In this article, we're going to focus on filtering a table by an array-column, but the video below covers a lot of other array-related functions:
+
+<iframe width="560" height="315" src="https://www.youtube.com/embed/JKHAdCFtYDg?si=OqS3ry1LFrOlF8Iy" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share" referrerpolicy="strict-origin-when-cross-origin" allowfullscreen></iframe>
+
+## Example
+
+We'll use an example of a table with two columns `tags_string` and `tags_int` that contain an array of strings and integers respectively.
+
+- Create a sample database and table.
+
+```sql
+CREATE DATABASE db1;
+```
+
+- Create a sample table
+
+```sql
+CREATE TABLE db1.tags_table
+(
+    `id` UInt64,
+    `tags_string` Array(String),
+    `tags_int` Array(UInt64),
+)
+ENGINE = MergeTree
+ORDER BY id;
+```
+
+- Insert some sample data into the table.
+
+```sql
+INSERT INTO db1.tags_table VALUES (1, ['tag1', 'tag2', 'tag3'], [1, 2, 3]), (2, ['tag2', 'tag3', 'tag4'], [2, 3, 4]), (3, ['tag1', 'tag3', 'tag5'], [1, 3, 5]);
+```
+
+Filter the table using the `has(arr, elem)` function to return the rows where the `arr` array contains the `elem` element.
+
+Filter the table to return the rows where the `tags_string` array contains the `tag1` element.
+
+```sql
+SELECT * FROM db1.tags_table WHERE has(tags_string, 'tag1');
+```
+
+```text
+┌─id─┬─tags_string────────────┬─tags_int─┐
+│  1 │ ['tag1','tag2','tag3'] │ [1,2,3]  │
+│  3 │ ['tag1','tag3','tag5'] │ [1,3,5]  │
+└────┴────────────────────────┴──────────┘
+```
+
+Use the `hasAll(arr, elems)` function to return the rows where all the elements in the `elems` array are present in the `arr` array.
+
+Filter the table to return the rows where all the elements in the `tags_string` array are present in the `['tag1', 'tag2']` array.
+
+```sql
+SELECT * FROM db1.tags_table WHERE hasAll(tags_string, ['tag1', 'tag2']);
+```
+
+```text
+┌─id─┬─tags_string────────────┬─tags_int─┐
+│  1 │ ['tag1','tag2','tag3'] │ [1,2,3]  │
+└────┴────────────────────────┴──────────┘
+```
+
+Use the `hasAny(arr, elems)` function to return the rows where at least one element in the `elems` array is present in the `arr` array.
+
+Filter the table to return the rows where at least one element in the `tags_string` array is present in the `['tag1', 'tag2']` array.
+
+```sql
+SELECT * FROM db1.tags_table WHERE hasAny(tags_string, ['tag1', 'tag2']);
+```
+
+```text
+┌─id─┬─tags_string────────────┬─tags_int─┐
+│  1 │ ['tag1','tag2','tag3'] │ [1,2,3]  │
+│  2 │ ['tag2','tag3','tag4'] │ [2,3,4]  │
+│  3 │ ['tag1','tag3','tag5'] │ [1,3,5]  │
+└────┴────────────────────────┴──────────┘
+```
+
+We can use a lambda function to filter the table using the `arrayExists(lambda, arr)` function. 
+
+Filter the table to return the rows where at least one element in the `tags_int` array is greater than 3.
+
+```sql
+SELECT * FROM db1.tags_table WHERE arrayExists(x -> x > 3, tags_int);
+```
+
+```text
+┌─id─┬─tags_string────────────┬─tags_int─┐
+│  2 │ ['tag2','tag3','tag4'] │ [2,3,4]  │
+│  3 │ ['tag1','tag3','tag5'] │ [1,3,5]  │
+└────┴────────────────────────┴──────────┘
+```

knowledgebase/how-to-insert-all-rows-from-another-table.md

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+---
+title: How to insert all rows from one table to another?
+description: "Knowledgebase article on how to insert all rows from one table to another."
+date: 2024-12-17
+---
+
+## Introduction
+
+Sometimes you need to reingest all the data from one table to another.
+
+For example, you might want to reingest data from a staging table to a production table. This article shows how to do this using the `INSERT INTO` statement.
+
+## Example
+
+Below is a simple example on how it works and how to test:
+
+- Create a sample database
+
+```sql 
+CREATE DATABASE db1;
+```
+
+- Create a sample table
+
+```sql
+CREATE TABLE db1.source_table
+(
+    city VARCHAR,
+    country VARCHAR,
+    continent VARCHAR
+)
+engine = MergeTree()
+ORDER BY continent;
+```
+
+- Insert some data into the source table
+
+```sql
+INSERT INTO db1.source_table (city, country, continent)
+VALUES
+    ('New York', 'USA', 'North America'),
+    ('Tokyo', 'Japan', 'Asia'),
+    ('Berlin', 'Germany', 'Europe'),
+    ('Paris', 'France', 'Europe'),
+    ('Cairo', 'Egypt', 'Africa'),
+    ('Sydney', 'Australia', 'Australia');
+```
+
+- Check the number of rows in the source table
+
+```sql
+SELECT COUNT(*) FROM db1.source_table;
+```
+
+```text
+┌─count()─┐
+│      6  │
+└─────────┘
+```
+
+- Create a new table with the same structure as the source table.
+
+```sql
+CREATE TABLE db1.target_table AS db1.source_table;
+```
+
+- Insert all rows from the source table to the target table.
+
+```sql
+INSERT INTO db1.target_table SELECT * FROM db1.source_table;
+```
+
+- Check the number of rows in the target table
+
+```sql
+SELECT COUNT(*) FROM db1.target_table;
+```
+
+```text
+┌─count()─┐
+│      6  │
+└─────────┘
+```
+
+If you want to modify the structure of the new table, you can first display the structure of the source table.
+
+```sql
+SHOW CREATE TABLE db1.source_table;
+```
+
+Then create the new table with the modified structure. In our case we want to add a new column `population` to the target table.
+
+```sql
+CREATE TABLE db1.target_table_population
+(
+    `city` String,
+    `country` String,
+    `continent` String,
+    `population` UInt16,
+)
+ENGINE = MergeTree
+ORDER BY continent;
+```
+
+- Insert all rows from the source table to the target table, including the new column. The population field is set to 0 for all rows.
+
+```sql
+INSERT INTO db1.target_table_population (city, country, continent, population)
+SELECT city, country, continent, 0 FROM db1.source_table;
+```
+
+- Check the data in the target table
+
+```sql
+SELECT * FROM db1.target_table_population LIMIT 3;
+```
+
+```text
+┌─city──────┬─country───┬─continent──────┬─population─┐
+│ New York  │ USA       │ North America  │          0 │
+│ Tokyo     │ Japan     │ Asia           │          0 │
+│ Berlin    │ Germany   │ Europe         │          0 │
+└───────────┴───────────┴────────────────┴────────────┘
+```
+
+
+