oracle如何把两列值合并

SELECT column1 || column2 FROM table_name;

SELECT CONCAT(column1, column2) FROM table_name;

SELECT CASE
    WHEN column1 IS NOT NULL THEN column1
    WHEN column2 IS NOT NULL THEN column2
    ELSE NULL  -- 如果两列都为空，返回 NULL
END FROM table_name;

CREATE TABLE my_table (
    id INT PRIMARY KEY,
    first_name VARCHAR(255),
    last_name VARCHAR(255)
);

INSERT INTO my_table (id, first_name, last_name) VALUES
(1, 'John', 'Doe'),
(2, 'Jane', 'Smith'),
(3, NULL, 'Wilson');

SELECT first_name || last_name AS full_name FROM my_table;

+-----------+
| full_name |
+-----------+
| John Doe  |
| Jane Smith |
| Wilson    |  -- 返回 NULL，因为 first_name 为 NULL
+-----------+

SELECT CONCAT(first_name, ' ', last_name) AS full_name FROM my_table;

+-----------+
| full_name |
+-----------+
| John Doe  |
| Jane Smith |
| NULL Wilson |  -- 返回 NULL，因为 first_name 为 NULL
+-----------+

SELECT CASE
    WHEN first_name IS NOT NULL THEN first_name
    WHEN last_name IS NOT NULL THEN last_name
    ELSE NULL
END AS full_name FROM my_table;

+-----------+
| full_name |
+-----------+
| John Doe  |
| Jane Smith |
| Wilson    |  -- 返回 "Wilson"，因为 last_name 不为空
+-----------+