# group_concat and string_agg

Calculates a string from the non-`NULL` values in a group, separated by a delimiter.

`group_concat` uses a comma when the delimiter is omitted. `string_agg` is the two-argument alias and requires an explicit delimiter.

**Syntax**

```text
group_concat(value [, separator] [ORDER BY expression])
string_agg(value, separator [ORDER BY expression])
```

**Arguments**

* `value` — The value to concatenate. Non-text values are converted to their text representation. `NULL` values are skipped.
* `separator` — The text placed between contributing values. The default for `group_concat` is `','`. A `NULL` separator adds no bytes.

When the separator is an expression that varies by row, each contributing row after the first uses its own separator. No separator is written before the first value.

**Returned value**

A text string. Empty and all-`NULL` inputs return `NULL`. A contributing empty string produces an empty string, rather than `NULL`.

**Examples**

**Joining employee names**

```sql
CREATE TABLE employees (name TEXT);
INSERT INTO employees VALUES ('Grace'), ('Ada'), (NULL), ('Linus');
```

```sql
SELECT group_concat(name, ', ' ORDER BY name) AS names
FROM employees
FORMAT csv;
```

```text
"Ada, Grace, Linus"
```

**Using the default separator and an explicit separator**

```sql
CREATE TABLE items (value TEXT);
INSERT INTO items VALUES ('b'), ('a'), ('c');
```

```sql
SELECT group_concat(value ORDER BY value),
       string_agg(value, ' | ' ORDER BY value)
FROM items
FORMAT csv;
```

```text
"a,b,c","a | b | c"
```

**Removing duplicate values**

```sql
CREATE TABLE tags (value TEXT);
INSERT INTO tags VALUES ('sql'), ('rust'), ('sql');
```

```sql
SELECT group_concat(DISTINCT value ORDER BY value)
FROM tags
FORMAT csv;
```

```text
"rust,sql"
```