Add note about SELECT CAST(MeN AS CHAR) incompatibility (pingcap#13518)…

… (pingcap#13558)

data-type-numeric.md

@@ -146,6 +146,10 @@ DOUBLE PRECISION [(M,D)] [UNSIGNED] [ZEROFILL], REAL[(M,D)] [UNSIGNED] [ZEROFILL
 >
 > As in MySQL, the `DOUBLE` data type stores approximate values. For values such as currency, it is recommended to use the `DECIMAL` type instead.
 
+> **Note:**
+>
+> When TiDB converts a double-precision floating-point number represented in scientific notation to the `CHAR` type, the result is displayed inconsistently with that in MySQL. See [Cast functions and operators](/functions-and-operators/cast-functions-and-operators.md) for details.
+
 ## Fixed-point types
 
 TiDB supports all the MySQL floating-point types, including DECIMAL, and NUMERIC. For more information, [Fixed-Point Types (Exact Value) - DECIMAL, NUMERIC in MySQL](https://dev.mysql.com/doc/refman/5.7/en/fixed-point-types.html).

functions-and-operators/cast-functions-and-operators.md

@@ -5,12 +5,16 @@ summary: Learn about the cast functions and operators.
 
 # Cast Functions and Operators
 
-TiDB supports all of the [cast functions and operators](https://dev.mysql.com/doc/refman/5.7/en/cast-functions.html) available in MySQL 5.7.
+Cast functions and operators enable conversion of values from one data type to another. TiDB supports all of the [cast functions and operators](https://dev.mysql.com/doc/refman/5.7/en/cast-functions.html) available in MySQL 5.7.
+
+## List of cast functions and operators
 
 | Name                                     | Description                      |
 | ---------------------------------------- | -------------------------------- |
 | [`BINARY`](https://dev.mysql.com/doc/refman/5.7/en/cast-functions.html#operator_binary) | Cast a string to a binary string |
 | [`CAST()`](https://dev.mysql.com/doc/refman/5.7/en/cast-functions.html#function_cast) | Cast a value as a certain type   |
 | [`CONVERT()`](https://dev.mysql.com/doc/refman/5.7/en/cast-functions.html#function_convert) | Cast a value as a certain type   |
 
-Cast functions and operators enable conversion of values from one data type to another.
+> **Note:**
+>
+> TiDB and MySQL display inconsistent results for `SELECT CAST(MeN AS CHAR)` (or its equivalent form `SELECT CONVERT(MeM, CHAR)`), where `MeN` represents a double-precision floating-point number in scientific notation. MySQL displays the complete numeric value when `-15 <= N <= 14` and the scientific notation when `N < -15` or `N > 14`. However, TiDB always displays the complete numeric value. For example, MySQL displays the result of `SELECT CAST(3.1415e15 AS CHAR)` as `3.1415e15`, while TiDB displays the result as `3141500000000000`.