### BOOL_AND Function
```sql
bool_and([distinct] expr)
```
#### Function Description
The BOOL_AND function is used to determine whether all values in a set of boolean values (expr) are true. If all values are true, it returns true; otherwise, it returns false. Using the distinct keyword can remove duplicates from the result to ensure each value is only counted once. Null values are not included in the calculation.

#### Parameter Description
- expr (required): The boolean expression that needs to be logically ANDed.

#### Return Type
- Returns a boolean value, true or false.

#### Usage Scenarios
1. Determine whether a set of data all meets a certain condition.
2. In multi-table join queries, used to connect multiple conditions.
3. Filter data to retain only records that meet all conditions.

#### Example
```sql
-- Example 1: Basic Usage
SELECT bool_and(condition) FROM table1;

-- Example 2: Use the distinct keyword to remove duplicates
SELECT bool_and(DISTINCT condition) FROM table1;

-- Example 3: Combine multi-table join queries
SELECT bool_and(t1.condition AND t2.condition) FROM table1 t1
JOIN table2 t2 ON t1.id = t2.id;

-- Example 4: Filter specific columns
SELECT * FROM table1 WHERE bool_and(col1, col2, col3);
```
#### Notes
- When the input expression is not of Boolean type, the BOOL_AND function will return an error.
- If the expression contains null values, the null values will not be included in the calculation.
- When using the DISTINCT keyword, ensure that there are values in the expression that can be deduplicated, otherwise the result may not be as expected.

Through the above content, you can better understand the purpose and usage of the BOOL_AND function, and thus apply it flexibly in actual queries.