# String Expressions
__*ANSI_QUOTES SQL mode schema*__
+ When using string expressions
  + Quotation marks are required
    + Use `'`  or `"`  quotation marks
    + `ANSI_QUOTES` The SQL pattern interprets characters in double quotes as identifiers
      + For example: `SELECT "string_literal";` 
    + Single quotes should be preferred for portability

------------------------------------------------------------
See the example `ANSI_QUOTES sql_mode` , and note the following behavior `select "aaa";` :

```
tidb> select 'AAA';
+-----+
| AAA |
+-----+
| AAA |
+-----+
1 row in set (0.00 sec)

tidb> select "aaa";
+-----+
| aaa |
+-----+
| aaa |
+-----+
1 row in set (0.00 sec)

tidb> set sql_mode=ansi_quotes;
Query OK, 0 rows affected (0.00 sec)

tidb> select 'AAA';
+-----+
| AAA |
+-----+
| AAA |
+-----+
1 row in set (0.00 sec)

tidb> select "aaa";
ERROR 1054 (42S22): Unknown column 'aaa' in 'field list'
```