# 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 instead of string literals 
    + Single quotes should be preferred

```
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'
```

------------------------------------------------------------
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'
```