PHP / MySQL联合有两个选择

Question

What's wrong with my statement? I've tried everything I can think of.

$stmt = $mysqli->prepare("SELECT host_name, review_title FROM lhr_reviews
              UNION
              SELECT host_url FROM lhr_hostinfo WHERE host_name = ?
              ORDER BY host_name");

$stmt->bind_param("s", $id);
$stmt->execute(); 
$res = $stmt->get_result();

while($row = $res->fetch_assoc()) {
  $list .=  "<li><a href='".$row['host_url']."'>".substr($row['review_title'],0,20)."</a></li>";
}

Answer 1

The columns that are selected from the two queries must match each others, in the number of columns and in the corresponding data types:

The column names from the first SELECT statement are used as the column names for the results returned. Selected columns listed in corresponding positions of each SELECT statement should have the same data type. (For example, the first column selected by the first statement should have the same type as the first column selected by the other statements.)

In your query, the first select selects two columns, where the second one selects only one column. This is not true, you can select empty string in the second select as a work around. The other issue is the ORDER BY host_name you can add it this way, you have to put these queries inside a subquery and order in the outer one like this:

SELECT *
FROM
(
    SELECT host_name, review_title FROM lhr_reviews
    UNION
    SELECT host_url, ''            FROM lhr_hostinfo WHERE host_name = ?
) AS sub
ORDER BY host_name;

Answer 2

you shoud try this query..

SELECT host_name, review_title,host_url FROM lhr_reviews
LEFT JOIIN lhr_hostinfo
    on(host_name = ?)
ORDER BY host_name

Answer 3

Problem in UNION . Selected columns listed in corresponding positions of each SELECT statement should have the same data type.

Try this:

$stmt = $mysqli->prepare(
             "SELECT * FROM
              (SELECT host_name, review_title FROM lhr_reviews
              UNION
              SELECT host_url, NULL FROM lhr_hostinfo WHERE host_name = ?) A
              ORDER BY host_name");

UNION Syntax

Answer 4

Actual problem is inside query because in first query you get two fields and at second query you getting single field and union says column must match including type of column and number of column.

Rules are.

Here are some rules that govern the way UNION operator is used in a query.

1. In a UNION query, there are at least two SELECT statements.

2. The two SELECT statements must have the same number of columns and the the columns must have compatible data types.

3. The column headings in each of the SELECT statements do not have to have the same name. The column headings in the result of a UNION query are always taken from the first SELECT statement.

4. If you want to sort the result set of the UNION operation, you can only put an ORDER BY clause after the last SELECT statement. ORDER BY clause can't be specified in any other SELECT statements in the UNION query.

5. The column(s) used in ORDER BY clause can only be taken from the first SELECT statement.

6. If you don't specify an ORDER BY clause in the UNION query, the result set is always sorted by the first column.

7. If you use UNION ALL, the entire result set from the second SELECT statement is appended to the first SELECT statement. In this case, there could be duplicate records in the unioned result set.

8. If you only use UNION, MySQL removes duplicate rows from the final result set.