string-functions: add instrunctions for LEFT() and LENGTH()

[takaidohigasi]

First-time contributors' checklist

• I've signed Contributor License Agreement that's required for repo owners to accept my contribution.

What is changed, added or deleted? (Required)

Fix #15491

This PR adds instructions for the LEFT() and LENGTH function. (one of the doc dash issues)

Which TiDB version(s) do your changes apply to? (Required)

Tips for choosing the affected version(s):

By default, CHOOSE MASTER ONLY so your changes will be applied to the next TiDB major or minor releases. If your PR involves a product feature behavior change or a compatibility change, CHOOSE THE AFFECTED RELEASE BRANCH(ES) AND MASTER.

For details, see tips for choosing the affected versions.

• master (the latest development version)
• v7.6 (TiDB 7.6 versions)
• v7.5 (TiDB 7.5 versions)
• v7.4 (TiDB 7.4 versions)
• v7.3 (TiDB 7.3 versions)
• v7.1 (TiDB 7.1 versions)
• v6.5 (TiDB 6.5 versions)
• v6.1 (TiDB 6.1 versions)
• v5.4 (TiDB 5.4 versions)
• v5.3 (TiDB 5.3 versions)
• v5.2 (TiDB 5.2 versions)
• v5.1 (TiDB 5.1 versions)
• v5.0 (TiDB 5.0 versions)

What is the related PR or file link(s)?

• This PR is translated from:
• Other reference link(s):

Do your changes match any of the following descriptions?

• Delete files
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

[CLAassistant]

All committers have signed the CLA.

[ti-chi-bot]

Welcome @takaidohigasi!

It looks like this is your first PR to pingcap/docs 🎉.

I'm the bot to help you request reviewers, add labels and more, See available commands.

We want to make sure your contribution gets all the attention it needs!



Thank you, and welcome to pingcap/docs. 😃

[takaidohigasi]

[FYI] link for .md
https://github.com/takaidohigasi/docs/blob/master/functions-and-operators/string-functions.md

[dveeden]

The content looks very good. I think it explains the normal case as well as common corner cases.

I think the SQL statement and output should be in separate blocks like this:

```sql
SELECT 1
```
```
+---+
| 1 |
+---+
| 1 |
+---+
1 row in set (0.01 sec)
```

Maybe the text for LENGTH() should have a reference to CHAR_LENGTH()?

mysql> SELECT LENGTH('Café');
+-----------------+
| LENGTH('Café')  |
+-----------------+
|               5 |
+-----------------+
1 row in set (0.00 sec)

mysql> SELECT CHAR_LENGTH('Café');
+----------------------+
| CHAR_LENGTH('Café')  |
+----------------------+
|                    4 |
+----------------------+
1 row in set (0.00 sec)

There might be some more wording and formatting things that need to be done.

[takaidohigasi]

The content looks very good. I think it explains the normal case as well as common corner cases.

I think the SQL statement and output should be in separate blocks like this:

```sql
SELECT 1

+---+
| 1 |
+---+
| 1 |
+---+
1 row in set (0.01 sec)

I will follow the documentation rule.
I followed existing example on the file and there are some similar documents, and I'm wondering what to do.

docs/functions-and-operators/string-functions.md

Lines 33 to 41 in 611cdde

	```sql
	SELECT ASCII('A');
	+------------+
	| ASCII('A') |
	+------------+
	| 65 |
	+------------+
	```

https://github.com/pingcap/docs/blob/611cddeb452ec2a652b1a644e36079fa6e1253a4/functions-and-operators/precision-math.md#rounding-behavior

Maybe the text for LENGTH() should have a reference to CHAR_LENGTH()?

mysql> SELECT LENGTH('Café');
+-----------------+
| LENGTH('Café')  |
+-----------------+
|               5 |
+-----------------+
1 row in set (0.00 sec)

mysql> SELECT CHAR_LENGTH('Café');
+----------------------+
| CHAR_LENGTH('Café')  |
+----------------------+
|                    4 |
+----------------------+
1 row in set (0.00 sec)

thanks, OK, I will add for this

[SeaRise]

lgtm

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-01-10 07:35:48.71467176 +0000 UTC m=+428738.298925432: ☑️ agreed by dveeden.
• 2024-01-29 07:33:49.576557659 +0000 UTC m=+1378071.140855364: ☑️ agreed by hfxsd.

[qiancai]

/approve

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: qiancai

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [qiancai]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[qiancai]

/test pull-verify