Fix a batch of broken links (pingcap#2647)

Author: CaitinChen

functions-and-operators/expressions-pushed-down.md

@@ -14,11 +14,11 @@ When TiDB reads data from TiKV, TiDB tries to push down some expressions (includ
 | Expression Type | Operations |
 | :-------------- | :------------------------------------- |
 | [Logical operators](/functions-and-operators/operators.md#logical-operators) | AND (&&), OR (||), NOT (!) |
-| [Comparison functions and operators](/functions-and-operators/operators.md#comparison-functions-and-operators) | `<`, `<=`, `=`, `!=` (`<>`), `>`, `>=`, [`<=>`](https://v3.1.mysql.com/doc/refman/5.7/en/comparison-operators.html#operator_equal-to), [`IN()`](https://v3.1.mysql.com/doc/refman/5.7/en/comparison-operators.html#function_in), IS NULL, LIKE, IS TRUE, IS FALSE, [`COALESCE()`](https://v3.1.mysql.com/doc/refman/5.7/en/comparison-operators.html#function_coalesce) |
-| [Numeric functions and operators](/functions-and-operators/numeric-functions-and-operators.md) | +, -, *, /, [`ABS()`](https://v3.1.mysql.com/doc/refman/5.7/en/mathematical-functions.html#function_abs), [`CEIL()`](https://v3.1.mysql.com/doc/refman/5.7/en/mathematical-functions.html#function_ceil), [`CEILING()`](https://v3.1.mysql.com/doc/refman/5.7/en/mathematical-functions.html#function_ceiling), [`FLOOR()`](https://v3.1.mysql.com/doc/refman/5.7/en/mathematical-functions.html#function_floor) |
-| [Control flow functions](/functions-and-operators/control-flow-functions.md) | [`CASE`](https://v3.1.mysql.com/doc/refman/5.7/en/control-flow-functions.html#operator_case), [`IF()`](https://v3.1.mysql.com/doc/refman/5.7/en/control-flow-functions.html#function_if), [`IFNULL()`](https://v3.1.mysql.com/doc/refman/5.7/en/control-flow-functions.html#function_ifnull) |
+| [Comparison functions and operators](/functions-and-operators/operators.md#comparison-functions-and-operators) | `<`, `<=`, `=`, `!=` (`<>`), `>`, `>=`, [`<=>`](https://dev.mysql.com/doc/refman/5.7/en/comparison-operators.html#operator_equal-to), [`IN()`](https://dev.mysql.com/doc/refman/5.7/en/comparison-operators.html#function_in), IS NULL, LIKE, IS TRUE, IS FALSE, [`COALESCE()`](https://dev.mysql.com/doc/refman/5.7/en/comparison-operators.html#function_coalesce) |
+| [Numeric functions and operators](/functions-and-operators/numeric-functions-and-operators.md) | +, -, *, /, [`ABS()`](https://dev.mysql.com/doc/refman/5.7/en/mathematical-functions.html#function_abs), [`CEIL()`](https://dev.mysql.com/doc/refman/5.7/en/mathematical-functions.html#function_ceil), [`CEILING()`](https://dev.mysql.com/doc/refman/5.7/en/mathematical-functions.html#function_ceiling), [`FLOOR()`](https://dev.mysql.com/doc/refman/5.7/en/mathematical-functions.html#function_floor) |
+| [Control flow functions](/functions-and-operators/control-flow-functions.md) | [`CASE`](https://dev.mysql.com/doc/refman/5.7/en/control-flow-functions.html#operator_case), [`IF()`](https://dev.mysql.com/doc/refman/5.7/en/control-flow-functions.html#function_if), [`IFNULL()`](https://dev.mysql.com/doc/refman/5.7/en/control-flow-functions.html#function_ifnull) |
 | [JSON functions](/functions-and-operators/json-functions.md) | [JSON_TYPE(json_val)][json_type],<br/> [JSON_EXTRACT(json_doc, path[, path] ...)][json_extract],<br/> [JSON_UNQUOTE(json_val)][json_unquote],<br/> [JSON_OBJECT(key, val[, key, val] ...)][json_object],<br/> [JSON_ARRAY([val[, val] ...])][json_array],<br/> [JSON_MERGE(json_doc, json_doc[, json_doc] ...)][json_merge],<br/> [JSON_SET(json_doc, path, val[, path, val] ...)][json_set],<br/> [JSON_INSERT(json_doc, path, val[, path, val] ...)][json_insert],<br/> [JSON_REPLACE(json_doc, path, val[, path, val] ...)][json_replace],<br/> [JSON_REMOVE(json_doc, path[, path] ...)][json_remove] |
-| [Date and time functions](/functions-and-operators/date-and-time-functions.md) | [`DATE_FORMAT()`](https://v3.1.mysql.com/doc/refman/5.7/en/date-and-time-functions.html#function_date-format)  |
+| [Date and time functions](/functions-and-operators/date-and-time-functions.md) | [`DATE_FORMAT()`](https://dev.mysql.com/doc/refman/5.7/en/date-and-time-functions.html#function_date-format)  |
 
 ## Blacklist specific expressions
 
@@ -125,24 +125,24 @@ tidb> explain select * from t where a < 2 and a > 2;
 | IS TRUE | istrue |
 | IS FALSE | isfalse |
 
-[json_extract]: https://v3.1.mysql.com/doc/refman/5.7/en/json-search-functions.html#function_json-extract
-[json_short_extract]: https://v3.1.mysql.com/doc/refman/5.7/en/json-search-functions.html#operator_json-column-path
-[json_short_extract_unquote]: https://v3.1.mysql.com/doc/refman/5.7/en/json-search-functions.html#operator_json-inline-path
-[json_unquote]: https://v3.1.mysql.com/doc/refman/5.7/en/json-modification-functions.html#function_json-unquote
-[json_type]: https://v3.1.mysql.com/doc/refman/5.7/en/json-attribute-functions.html#function_json-type
-[json_set]: https://v3.1.mysql.com/doc/refman/5.7/en/json-modification-functions.html#function_json-set
-[json_insert]: https://v3.1.mysql.com/doc/refman/5.7/en/json-modification-functions.html#function_json-insert
-[json_replace]: https://v3.1.mysql.com/doc/refman/5.7/en/json-modification-functions.html#function_json-replace
-[json_remove]: https://v3.1.mysql.com/doc/refman/5.7/en/json-modification-functions.html#function_json-remove
-[json_merge]: https://v3.1.mysql.com/doc/refman/5.7/en/json-modification-functions.html#function_json-merge
-[json_merge_preserve]: https://v3.1.mysql.com/doc/refman/5.7/en/json-modification-functions.html#function_json-merge-preserve
-[json_object]: https://v3.1.mysql.com/doc/refman/5.7/en/json-creation-functions.html#function_json-object
-[json_array]: https://v3.1.mysql.com/doc/refman/5.7/en/json-creation-functions.html#function_json-array
-[json_keys]: https://v3.1.mysql.com/doc/refman/5.7/en/json-search-functions.html#function_json-keys
-[json_length]: https://v3.1.mysql.com/doc/refman/5.7/en/json-attribute-functions.html#function_json-length
-[json_valid]: https://v3.1.mysql.com/doc/refman/5.7/en/json-attribute-functions.html#function_json-valid
-[json_quote]: https://v3.1.mysql.com/doc/refman/5.7/en/json-creation-functions.html#function_json-quote
-[json_contains]: https://v3.1.mysql.com/doc/refman/5.7/en/json-search-functions.html#function_json-contains
-[json_contains_path]: https://v3.1.mysql.com/doc/refman/5.7/en/json-search-functions.html#function_json-contains-path
-[json_arrayagg]: https://v3.1.mysql.com/doc/refman/5.7/en/group-by-functions.html#function_json-arrayagg
-[json_depth]: https://v3.1.mysql.com/doc/refman/5.7/en/json-attribute-functions.html#function_json-depth
+[json_extract]: https://dev.mysql.com/doc/refman/5.7/en/json-search-functions.html#function_json-extract
+[json_short_extract]: https://dev.mysql.com/doc/refman/5.7/en/json-search-functions.html#operator_json-column-path
+[json_short_extract_unquote]: https://dev.mysql.com/doc/refman/5.7/en/json-search-functions.html#operator_json-inline-path
+[json_unquote]: https://dev.mysql.com/doc/refman/5.7/en/json-modification-functions.html#function_json-unquote
+[json_type]: https://dev.mysql.com/doc/refman/5.7/en/json-attribute-functions.html#function_json-type
+[json_set]: https://dev.mysql.com/doc/refman/5.7/en/json-modification-functions.html#function_json-set
+[json_insert]: https://dev.mysql.com/doc/refman/5.7/en/json-modification-functions.html#function_json-insert
+[json_replace]: https://dev.mysql.com/doc/refman/5.7/en/json-modification-functions.html#function_json-replace
+[json_remove]: https://dev.mysql.com/doc/refman/5.7/en/json-modification-functions.html#function_json-remove
+[json_merge]: https://dev.mysql.com/doc/refman/5.7/en/json-modification-functions.html#function_json-merge
+[json_merge_preserve]: https://dev.mysql.com/doc/refman/5.7/en/json-modification-functions.html#function_json-merge-preserve
+[json_object]: https://dev.mysql.com/doc/refman/5.7/en/json-creation-functions.html#function_json-object
+[json_array]: https://dev.mysql.com/doc/refman/5.7/en/json-creation-functions.html#function_json-array
+[json_keys]: https://dev.mysql.com/doc/refman/5.7/en/json-search-functions.html#function_json-keys
+[json_length]: https://dev.mysql.com/doc/refman/5.7/en/json-attribute-functions.html#function_json-length
+[json_valid]: https://dev.mysql.com/doc/refman/5.7/en/json-attribute-functions.html#function_json-valid
+[json_quote]: https://dev.mysql.com/doc/refman/5.7/en/json-creation-functions.html#function_json-quote
+[json_contains]: https://dev.mysql.com/doc/refman/5.7/en/json-search-functions.html#function_json-contains
+[json_contains_path]: https://dev.mysql.com/doc/refman/5.7/en/json-search-functions.html#function_json-contains-path
+[json_arrayagg]: https://dev.mysql.com/doc/refman/5.7/en/group-by-functions.html#function_json-arrayagg
+[json_depth]: https://dev.mysql.com/doc/refman/5.7/en/json-attribute-functions.html#function_json-depth