add more explains and examples for SELECT ... INTO OUTFILE statements (#15104)

sql-statements/sql-statement-select.md

@@ -13,10 +13,6 @@ The `SELECT` statement is used to read data from TiDB.
 
 ![SelectStmt](/media/sqlgram/SelectStmt.png)
 
-> **Note:**
->
-> The `SELECT ... INTO OUTFILE` statement is only applicable to TiDB Self-Hosted and not available on [TiDB Cloud](https://docs.pingcap.com/tidbcloud/).
-
 **FromDual:**
 
 ![FromDual](/media/sqlgram/FromDual.png)
@@ -117,6 +113,8 @@ TableSampleOpt ::=
 
 ## Examples
 
+### SELECT
+
 ```sql
 mysql> CREATE TABLE t1 (id INT NOT NULL PRIMARY KEY AUTO_INCREMENT, c1 INT NOT NULL);
 Query OK, 0 rows affected (0.11 sec)
@@ -158,12 +156,86 @@ mysql> SELECT AVG(s_quantity), COUNT(s_quantity) FROM stock;
 
 The above example uses data generated with `tiup bench tpcc prepare`. The first query shows the use of `TABLESAMPLE`.
 
+### SELECT ... INTO OUTFILE
+
+The `SELECT ... INTO OUTFILE` statement is used to write the result of a query to a file.
+
+> **Note:**
+>
+> - This statement is only applicable to TiDB Self-Hosted and not available on [TiDB Cloud](https://docs.pingcap.com/tidbcloud/).
+> - This statement does not support writing query results to any [external storages](https://docs.pingcap.com/tidb/stable/backup-and-restore-storages) such as Amazon S3 or GCS.
+
+In the statement, you can specify the format of the output file by using the following clauses:
+
+- `FIELDS TERMINATED BY`: specifies the field delimiter in the file. For example, you can specify it as `','` to output comma-separated values (CSV) or `'\t'` to output tab-separated values (TSV).
+- `FIELDS ENCLOSED BY`: specifies the enclosing character that wraps around each field in the file.
+- `LINES TERMINATED BY`: specifies the line terminator in the file, if you want to end a line with a certain character.
+
+Assume that there is a table `t` with three columns as follows:
+
+```sql
+mysql> CREATE TABLE t (a INT, b VARCHAR(10), c DECIMAL(10,2));
+Query OK, 0 rows affected (0.02 sec)
+
+mysql> INSERT INTO t VALUES (1, 'a', 1.1), (2, 'b', 2.2), (3, 'c', 3.3);
+Query OK, 3 rows affected (0.01 sec)
+```
+
+The following examples show how to use the `SELECT ... INTO OUTFILE` statement to write the query result to a file.
+
+**Example 1:**
+
+```sql
+mysql> SELECT * FROM t INTO OUTFILE '/tmp/tmp_file1';
+Query OK, 3 rows affected (0.00 sec)
+```
+
+In this example, you can find the query result in `/tmp/tmp_file1` as follows:
+
+```
+1       a       1.10
+2       b       2.20
+3       c       3.30
+```
+
+**Example 2:**
+
+```sql
+mysql> SELECT * FROM t INTO OUTFILE '/tmp/tmp_file2' FIELDS TERMINATED BY ',' ENCLOSED BY '"';
+Query OK, 3 rows affected (0.00 sec)
+```
+
+In this example, you can find the query result in `/tmp/tmp_file2` as follows:
+
+```
+"1","a","1.10"
+"2","b","2.20"
+"3","c","3.30"
+```
+
+**Example 3:**
+
+```sql
+mysql> SELECT * FROM t INTO OUTFILE '/tmp/tmp_file3'
+    -> FIELDS TERMINATED BY ',' ENCLOSED BY '\'' LINES TERMINATED BY '<<<\n';
+Query OK, 3 rows affected (0.00 sec)
+```
+
+In this example, you can find the query result in `/tmp/tmp_file3` as follows:
+
+```
+'1','a','1.10'<<<
+'2','b','2.20'<<<
+'3','c','3.30'<<<
+```
+
 ## MySQL compatibility
 
 - The syntax `SELECT ... INTO @variable` is not supported.
 - The syntax `SELECT ... GROUP BY ... WITH ROLLUP` is not supported.
+- The syntax `SELECT ... INTO DUMPFILE` is not supported.
 - The syntax `SELECT .. GROUP BY expr` does not imply `GROUP BY expr ORDER BY expr` as it does in MySQL 5.7. TiDB instead matches the behavior of MySQL 8.0 and does not imply a default order.
-- The syntax `SELECT ... TABLESAMPLE ...` is a TiDB extension and not supported by MySQL.
+- The syntax `SELECT ... TABLESAMPLE ...` is a TiDB extension designed for compatibility with other database systems and the [ISO/IEC 9075-2](https://standards.iso.org/iso-iec/9075/-2/ed-6/en/) standard, but currently it is not supported by MySQL.
 
 ## See also