sql: add sequence doc

reference/sql/statements/create-sequence.md

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+---
+title: CREATE SEQUENCE
+summary: CREATE SEQUENCE 在 TiDB 中的使用概况
+category: reference
+---
+
+# CREATE SEQUENCE
+
+`CREATE SEQUENCE` 语句用于在 TiDB 中创建 SEQUENCE 对象。SEQUENCE 是一种与表、视图对象平级的数据库对象，用于进行自定义的序列化 id 生成。
+
+## 语法图
+
+**CreateSequenceStmt:**
+
+![CreateSequenceStmt](/media/sqlgram/CreateSequenceStmt.png)
+
+**OptTemporary:**
+
+![OptTemporary](/media/sqlgram/OptTemporary.png)
+
+**IfNotExists:**
+
+![IfNotExists](/media/sqlgram/IfNotExists.png)
+
+**TableName:**
+
+![TableName](/media/sqlgram/TableName.png)
+
+**CreateSequenceOptionListOpt:**
+
+![CreateSequenceOptionListOpt](/media/sqlgram/CreateSequenceOptionListOpt.png)
+
+**SequenceOption:**
+
+![SequenceOption](/media/sqlgram/SequenceOption.png)
+
+**CreateTableOptionListOpt:**
+
+![CreateTableOptionListOpt](/media/sqlgram/CreateTableOptionListOpt.png)
+
+## 语法说明
+
+```
+CREATE [TEMPORARY] SEQUENCE [IF NOT EXISTS] sequence_name
+    [ INCREMENT [ BY | = ] increment ]
+    [ MINVALUE [=] minvalue | NO MINVALUE | NOMINVALUE ]
+    [ MAXVALUE [=] maxvalue | NO MAXVALUE | NOMAXVALUE ]
+    [ START [ WITH | = ] start ]
+    [ CACHE [=] cache | NOCACHE | NO CACHE]
+    [ CYCLE | NOCYCLE | NO CYCLE]
+    [ ORDER | NOORDER | NO ORDER]
+    [table_options]
+```
+
+## 参数说明
+
+参数 | 默认值 | 描述  
+ -|-|-
+INCREMENT | 1 | 指定序列的步长，其正负值可以控制 SEQUENCE 的增长方向。
+MINVALUE | 1 / -9223372036854775807 | 指定序列的最小值，当 INCREMENT > 0 时，默认值为 1；当 INCREMENT < 0 时，默认值为 -9223372036854775807。
+MAXVALUE | 9223372036854775806 / -1 | 指定序列的最大值，当 INCREMENT > 0 时，默认值为 9223372036854775806；当 INCREMENT < 0 时，默认值为 -1。
+START | MINVALUE / MAXVALUE | 指定序列的初始值，当 INCREMENT > 0 时，默认值为 MINVALUE; 当 INCREMENT < 0 时，默认值为 MAXVALUE。
+CACHE | 1000 | 指定每个 TiDB 本地缓存序列的大小，默认值为 1000。
+CYCLE | false | 指定序列用完之后的是否循环使用。在 CYCLE 的情况下，当 INCREMENT > 0 时，默认值为 MINVALUE；当 INCREMENT < 0 时，默认值为 MAXVALUE。
+
+## SEQUENCE 函数
+
+SEQUENCE 的使用主要通过表达式函数来操纵
+
+**NEXTVAL / NEXT VALUE FOR**
+
+本质上都是 nextval() 函数, 获取 SEQUENCE 对象的下一个有效值，其参数为 SEQUENCE 序列的 identifier。
+
+**LASTVAL**
+
+lastval() 函数，用于获取本会话上一个使用过的值，如果没有则为 NULL, 其参数为 SEQUENCE 序列的 identifier。
+
+**SETVAL**
+
+setval() 函数，用于设置序列的增长，其第一参数为 SEQUENCE 序列的 identifier，第二个参数为 num。
+
+> 注意：在 TiDB SEQUENCE 的实现中，setval 函数并不能改变序列增长的初始步调/循环步调，在 setval 之后只会返回符合步调规律的下一个有效的序列值。
+
+## 示例
+
+{{< copyable "sql" >}}
+
+```sql
+CREATE SEQUENCE seq;
+```
+
+```
+Query OK, 0 rows affected (0.06 sec)
+```
+
+{{< copyable "sql" >}}
+
+```sql
+SELECT nextval(seq);
+```
+
+```
++--------------+
+| nextval(seq) |
++--------------+
+|            1 |
++--------------+
+1 row in set (0.02 sec)
+```
+
+{{< copyable "sql" >}}
+
+```sql
+SELECT lastval(seq);
+```
+
+```
++--------------+
+| lastval(seq) |
++--------------+
+|            1 |
++--------------+
+1 row in set (0.02 sec)
+```
+
+{{< copyable "sql" >}}
+
+```sql
+SELECT setval(seq, 10);
+```
+
+```
++-----------------+
+| setval(seq, 10) |
++-----------------+
+|              10 |
++-----------------+
+1 row in set (0.01 sec)
+```
+
+{{< copyable "sql" >}}
+
+```sql
+SELECT next value for seq;
+```
+
+```
++--------------------+
+| next value for seq |
++--------------------+
+|                 11 |
++--------------------+
+1 row in set (0.00 sec)
+```
+
+{{< copyable "sql" >}}
+
+```sql
+CREATE SEQUENCE seq2 start 3 increment 2 minvalue 1 maxvalue 10 cache 3;
+```
+
+```
+Query OK, 0 rows affected (0.01 sec)
+```
+
+{{< copyable "sql" >}}
+
+```sql
+SELECT lastval(seq2);
+```
+
+```
++---------------+
+| lastval(seq2) |
++---------------+
+|          NULL |
++---------------+
+1 row in set (0.01 sec)
+```
+
+{{< copyable "sql" >}}
+
+```sql
+SELECT nextval(seq2);
+```
+
+```
++---------------+
+| nextval(seq2) |
++---------------+
+|             3 |
++---------------+
+1 row in set (0.00 sec)
+```
+
+{{< copyable "sql" >}}
+
+```sql
+SELECT setval(seq2, 6);
+```
+
+```
++-----------------+
+| setval(seq2, 6) |
++-----------------+
+|               6 |
++-----------------+
+1 row in set (0.00 sec)
+```
+
+{{< copyable "sql" >}}
+
+```sql
+SELECT next value for seq2;
+```
+
+```
++---------------------+
+| next value for seq2 |
++---------------------+
+|                   7 |
++---------------------+
+1 row in set (0.00 sec)
+```
+
+{{< copyable "sql" >}}
+
+```sql
+CRATE table t(a int default next value for seq2);
+```
+
+```
+Query OK, 0 rows affected (0.02 sec)
+```
+
+{{< copyable "sql" >}}
+
+```sql
+INSERT into t values();
+```
+
+```
+Query OK, 1 row affected (0.00 sec)
+```
+
+{{< copyable "sql" >}}
+
+```sql
+SELECT * from t;
+```
+
+```
++------+
+| a    |
++------+
+|    9 |
++------+
+1 row in set (0.00 sec)
+```
+
+{{< copyable "sql" >}}
+
+```sql
+INSERT into t values();
+```
+
+```
+ERROR 4135 (HY000): Sequence 'test.seq2' has run out
+```
+
+## MySQL 兼容性
+
+* MySQL 暂无 SEQUENCE 选项。TiDB Sequence 借鉴自 MariaDB，但是 setval 会保持原有的步调。
+
+## 另请参阅
+
+* [DROP SEQUENCE](/reference/sql/statements/drop-sequence.md)
+* [SHOW CREATE SEQUENCE](/reference/sql/statements/show-create-sequence.md)

reference/sql/statements/drop-sequence.md

@@ -0,0 +1,58 @@
+---
+title: DROP SEQUENCE
+summary: TiDB 数据库中 DROP SEQUENCE 的使用概况。
+category: reference
+---
+
+# DROP SEQUENCE
+
+`DROP SEQUENCE` 语句用于删除 SEQUENCE 对象。
+
+## 语法图
+
+**DropSequenceStmt:**
+
+![DropSequenceStmt](/media/sqlgram/DropSequenceStmt.png)
+
+**OptTemporary:**
+
+![OptTemporary](/media/sqlgram/OptTemporary.png)
+
+**IfExists:**
+
+![IfExists](/media/sqlgram/IfExists.png)
+
+**TableNameList:**
+
+![TableNameList](/media/sqlgram/TableNameList.png)
+
+## 示例
+
+{{< copyable "sql" >}}
+
+```sql
+DROP SEQUENCE seq;
+```
+
+```
+Query OK, 0 rows affected (0.10 sec)
+```
+
+{{< copyable "sql" >}}
+
+```sql
+DROP SEQUENCE seq, seq2;
+```
+
+```
+Query OK, 0 rows affected (0.03 sec)
+```
+
+## MySQL 兼容性
+
+* MySQL 暂无 SEQUENCE 选项。TiDB Sequence 借鉴自 MariaDB，但是 setval 会保持原有的步调。
+
+## 另请参阅
+
+* [CREATE SEQUENCE](/reference/sql/statements/create-sequence.md)
+* [SHOW CREATE SEQUENCE](/reference/sql/statements/show-create-sequence.md)

reference/sql/statements/show-create-sequence.md

@@ -0,0 +1,55 @@
+---
+title: SHOW CREATE SEQUENCE
+summary: TiDB 数据库中 SHOW CREATE SEQUENCE 的使用概况。
+category: reference
+---
+
+# SHOW CREATE SEQUENCE
+
+`SHOW CREATE SEQUENCE` 语句用于查看一个 SEQUENCE 的详细信息，类似于 `SHOW CREATE TABLE` 语句。
+
+## 语法图
+
+**ShowStmt:**
+
+![ShowStmt](/media/sqlgram/ShowStmt.png)
+
+**TableName:**
+
+![TableName](/media/sqlgram/TableName.png)
+
+## 示例
+
+{{< copyable "sql" >}}
+
+```sql
+CREATE TABLE seq;
+```
+
+```
+Query OK, 0 rows affected (0.03 sec)
+```
+
+{{< copyable "sql" >}}
+
+```sql
+SHOW CREATE SEQUENCE seq;
+```
+
+```
++-------+----------------------------------------------------------------------------------------------------------------------------+
+| Table | Create Table                                                                                                               |
++-------+----------------------------------------------------------------------------------------------------------------------------+
+| seq   | CREATE SEQUENCE `seq` start with 1 minvalue 1 maxvalue 9223372036854775806 increment by 1 cache 1000 nocycle ENGINE=InnoDB |
++-------+----------------------------------------------------------------------------------------------------------------------------+
+1 row in set (0.00 sec)
+```
+
+## MySQL 兼容性
+
+* MySQL 暂无 SEQUENCE 选项。TiDB Sequence 借鉴自 MariaDB，但是 setval 会保持原有的步调。
+
+## 另请参阅
+
+* [CREATE SEQUENCE](/reference/sql/statements/create-sequence.md)
+* [DROP SEQUENCE](/reference/sql/statements/drop-sequence.md)