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DOCAPI-6554: Extended syntax for LIMIT BY clause.
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BayoNet
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@ -17,7 +17,7 @@ SELECT [DISTINCT] expr_list
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[UNION ALL ...]
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[INTO OUTFILE filename]
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[FORMAT format]
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[LIMIT n BY columns]
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[LIMIT [offset_value, ]n BY columns]
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```
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All the clauses are optional, except for the required list of expressions immediately after SELECT.
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@ -46,7 +46,7 @@ The FINAL modifier can be used only for a SELECT from a CollapsingMergeTree tabl
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### SAMPLE Clause {#select-sample-clause}
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The `SAMPLE` clause allows for approximated query processing.
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The `SAMPLE` clause allows for approximated query processing.
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When data sampling is enabled, the query is not performed on all the data, but only on a certain fraction of data (sample). For example, if you need to calculate statistics for all the visits, it is enough to execute the query on the 1/10 fraction of all the visits and then multiply the result by 10.
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@ -67,11 +67,11 @@ The features of data sampling are listed below:
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For the `SAMPLE` clause the following syntax is supported:
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| SAMPLE Clause Syntax | Description |
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| ---------------- | --------- |
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| SAMPLE Clause Syntax | Description |
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| ---------------- | --------- |
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| `SAMPLE k` | Here `k` is the number from 0 to 1.</br>The query is executed on `k` fraction of data. For example, `SAMPLE 0.1` runs the query on 10% of data. [Read more](#select-sample-k)|
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| `SAMPLE n` | Here `n` is a sufficiently large integer.</br>The query is executed on a sample of at least `n` rows (but not significantly more than this). For example, `SAMPLE 10000000` runs the query on a minimum of 10,000,000 rows. [Read more](#select-sample-n) |
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| `SAMPLE k OFFSET m` | Here `k` and `m` are the numbers from 0 to 1.</br>The query is executed on a sample of `k` fraction of the data. The data used for the sample is offset by `m` fraction. [Read more](#select-sample-offset) |
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| `SAMPLE k OFFSET m` | Here `k` and `m` are the numbers from 0 to 1.</br>The query is executed on a sample of `k` fraction of the data. The data used for the sample is offset by `m` fraction. [Read more](#select-sample-offset) |
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#### SAMPLE k {#select-sample-k}
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@ -129,7 +129,7 @@ SELECT avg(Duration)
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FROM visits
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SAMPLE 10000000
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```
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#### SAMPLE k OFFSET m {#select-sample-offset}
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Here `k` and `m` are numbers from 0 to 1. Examples are shown below.
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@ -216,10 +216,10 @@ The next example uses the `LEFT ARRAY JOIN` clause:
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``` sql
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SELECT s, arr
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FROM arrays_test
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FROM arrays_test
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LEFT ARRAY JOIN arr;
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```
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```
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```
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┌─s───────────┬─arr─┐
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│ Hello │ 1 │
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│ Hello │ 2 │
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@ -228,7 +228,7 @@ LEFT ARRAY JOIN arr;
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│ World │ 5 │
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│ Goodbye │ 0 │
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└─────────────┴─────┘
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```
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```
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#### Using Aliases
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@ -240,7 +240,7 @@ FROM arrays_test
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ARRAY JOIN arr AS a;
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```
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```
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```
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┌─s─────┬─arr─────┬─a─┐
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│ Hello │ [1,2] │ 1 │
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│ Hello │ [1,2] │ 2 │
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@ -254,7 +254,7 @@ Using aliases, you can perform `ARRAY JOIN` with an external array. For example:
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``` sql
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SELECT s, arr_external
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FROM arrays_test
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FROM arrays_test
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ARRAY JOIN [1, 2, 3] AS arr_external;
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```
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@ -325,7 +325,7 @@ INSERT INTO nested_test
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VALUES ('Hello', [1,2], [10,20]), ('World', [3,4,5], [30,40,50]), ('Goodbye', [], []);
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```
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```
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```
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┌─s───────┬─nest.x──┬─nest.y─────┐
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│ Hello │ [1,2] │ [10,20] │
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│ World │ [3,4,5] │ [30,40,50] │
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@ -339,7 +339,7 @@ FROM nested_test
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ARRAY JOIN nest;
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```
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```
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```
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┌─s─────┬─nest.x─┬─nest.y─┐
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│ Hello │ 1 │ 10 │
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│ Hello │ 2 │ 20 │
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@ -357,7 +357,7 @@ FROM nested_test
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ARRAY JOIN `nest.x`, `nest.y`;
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```
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```
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```
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┌─s─────┬─nest.x─┬─nest.y─┐
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│ Hello │ 1 │ 10 │
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│ Hello │ 2 │ 20 │
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@ -375,7 +375,7 @@ FROM nested_test
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ARRAY JOIN `nest.x`;
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```
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```
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```
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┌─s─────┬─nest.x─┬─nest.y─────┐
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│ Hello │ 1 │ [10,20] │
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│ Hello │ 2 │ [10,20] │
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@ -393,7 +393,7 @@ FROM nested_test
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ARRAY JOIN nest AS n;
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```
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```
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```
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┌─s─────┬─n.x─┬─n.y─┬─nest.x──┬─nest.y─────┐
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│ Hello │ 1 │ 10 │ [1,2] │ [10,20] │
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│ Hello │ 2 │ 20 │ [1,2] │ [10,20] │
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@ -411,7 +411,7 @@ FROM nested_test
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ARRAY JOIN nest AS n, arrayEnumerate(`nest.x`) AS num;
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```
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```
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```
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┌─s─────┬─n.x─┬─n.y─┬─nest.x──┬─nest.y─────┬─num─┐
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│ Hello │ 1 │ 10 │ [1,2] │ [10,20] │ 1 │
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│ Hello │ 2 │ 20 │ [1,2] │ [10,20] │ 2 │
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@ -669,11 +669,53 @@ When external aggregation is enabled, if there was less than ` max_bytes_before_
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If you have an ORDER BY with a small LIMIT after GROUP BY, then the ORDER BY CLAUSE will not use significant amounts of RAM.
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But if the ORDER BY doesn't have LIMIT, don't forget to enable external sorting (`max_bytes_before_external_sort`).
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### LIMIT N BY Clause
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### LIMIT BY Clause
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LIMIT N BY COLUMNS selects the top N rows for each group of COLUMNS. LIMIT N BY is not related to LIMIT; they can both be used in the same query. The key for LIMIT N BY can contain any number of columns or expressions.
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The query with the `LIMIT n BY columns` clause selects the top `n` rows for each group of `columns`. `LIMIT BY` is not related to `LIMIT`, they can both be used in the same query. The key for `LIMIT BY` can contain any number of columns or expressions.
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Example:
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ClickHouse supports the following syntax:
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- `LIMIT [offset_value, ]n BY columns`
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- `LIMIT n OFFSET offset_value BY columns`
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The `OFFSET` value sets the number of rows which ClickHouse skips from the beginning of the output.
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**Examples**
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Sample table:
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```sql
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CREATE TABLE limit_by(id Int, val Int) ENGINE = Memory;
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INSERT INTO limit_by values(1, 10), (1, 11), (1, 12), (2, 20), (2, 21);
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```
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Queries:
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```sql
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SELECT * FROM limit_by ORDER BY id, val LIMIT 2 BY id
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```
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```text
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┌─id─┬─val─┐
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│ 1 │ 10 │
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│ 1 │ 11 │
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│ 2 │ 20 │
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│ 2 │ 21 │
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└────┴─────┘
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```
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```sql
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SELECT * FROM limit_by ORDER BY id, val LIMIT 1, 2 BY id
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```
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```text
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┌─id─┬─val─┐
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│ 1 │ 11 │
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│ 1 │ 12 │
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│ 2 │ 21 │
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└────┴─────┘
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```
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The `SELECT * FROM limit_by ORDER BY id, val LIMIT 2 OFFSET 1 BY id` query returns the same result.
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The following query returns the top 5 referrers for each `domain, device_type` pair, but not more than 100 rows (`LIMIT n BY + LIMIT`).
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``` sql
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SELECT
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@ -688,8 +730,6 @@ LIMIT 5 BY domain, device_type
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LIMIT 100
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```
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The query will select the top 5 referrers for each `domain, device_type` pair, but not more than 100 rows (`LIMIT n BY + LIMIT`).
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### HAVING Clause
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Allows filtering the result received after GROUP BY, similar to the WHERE clause.
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