Edit: class-wp-tax-query.php

<?php
/**
 * Taxonomy API: WP_Tax_Query class
 *
 * @package WordPress
 * @subpackage Taxonomy
 * @since 4.4.0
 */

/**
 * Core class used to implement taxonomy queries for the Taxonomy API.
 *
 * Used for generating SQL clauses that filter a primary query according to object
 * taxonomy terms.
 *
 * WP_Tax_Query is a helper that allows primary query classes, such as WP_Query, to filter
 * their results by object metadata, by generating `JOIN` and `WHERE` subclauses to be
 * attached to the primary SQL query string.
 *
 * @since 3.1.0
 */
#[AllowDynamicProperties]
class WP_Tax_Query {

/**
	 * Array of taxonomy queries.
	 *
	 * See WP_Tax_Query::__construct() for information on tax query arguments.
	 *
	 * @since 3.1.0
	 * @var array
	 */
	public $queries = array();

/**
	 * The relation between the queries. Can be one of 'AND' or 'OR'.
	 *
	 * @since 3.1.0
	 * @var string
	 */
	public $relation;

/**
	 * Standard response when the query should not return any rows.
	 *
	 * @since 3.2.0
	 * @var array<string, array<string>>
	 */
	private static $no_results = array(
		'join'  => array( '' ),
		'where' => array( '0 = 1' ),
	);

/**
	 * A flat list of table aliases used in the JOIN clauses.
	 *
	 * @since 4.1.0
	 * @var array
	 */
	protected $table_aliases = array();

/**
	 * Terms and taxonomies fetched by this query.
	 *
	 * We store this data in a flat array because they are referenced in a
	 * number of places by WP_Query.
	 *
	 * @since 4.1.0
	 * @var array
	 */
	public $queried_terms = array();

/**
	 * Database table that where the metadata's objects are stored (eg $wpdb->users).
	 *
	 * @since 4.1.0
	 * @var string
	 */
	public $primary_table;

/**
	 * Column in 'primary_table' that represents the ID of the object.
	 *
	 * @since 4.1.0
	 * @var string
	 */
	public $primary_id_column;

/**
	 * Constructor.
	 *
	 * @since 3.1.0
	 * @since 4.1.0 Added support for `$operator` 'NOT EXISTS' and 'EXISTS' values.
	 *
	 * @param array $tax_query {
	 *     Array of taxonomy query clauses.
	 *
	 *     @type string $relation Optional. The MySQL keyword used to join
	 *                            the clauses of the query. Accepts 'AND', or 'OR'. Default 'AND'.
	 *     @type array  ...$0 {
	 *         An array of first-order clause parameters, or another fully-formed tax query.
	 *
	 *         @type string           $taxonomy         Taxonomy being queried. Optional when field=term_taxonomy_id.
	 *         @type string|int|array $terms            Term or terms to filter by.
	 *         @type string           $field            Field to match $terms against. Accepts 'term_id', 'slug',
	 *                                                 'name', or 'term_taxonomy_id'. Default: 'term_id'.
	 *         @type string           $operator         MySQL operator to be used with $terms in the WHERE clause.
	 *                                                  Accepts 'AND', 'IN', 'NOT IN', 'EXISTS', 'NOT EXISTS'.
	 *                                                  Default: 'IN'.
	 *         @type bool             $include_children Optional. Whether to include child terms.
	 *                                                  Requires a $taxonomy. Default: true.
	 *     }
	 * }
	 */
	public function __construct( $tax_query ) {
		if ( isset( $tax_query['relation'] ) ) {
			$this->relation = $this->sanitize_relation( $tax_query['relation'] );
		} else {
			$this->relation = 'AND';
		}

$this->queries = $this->sanitize_query( $tax_query );
	}

/**
	 * Ensures the 'tax_query' argument passed to the class constructor is well-formed.
	 *
	 * Ensures that each query-level clause has a 'relation' key, and that
	 * each first-order clause contains all the necessary keys from `$defaults`.
	 *
	 * @since 4.1.0
	 *
	 * @param array $queries Array of queries clauses.
	 * @return array Sanitized array of query clauses.
	 */
	public function sanitize_query( $queries ) {
		$cleaned_query = array();

$defaults = array(
			'taxonomy'         => '',
			'terms'            => array(),
			'field'            => 'term_id',
			'operator'         => 'IN',
			'include_children' => true,
		);

foreach ( $queries as $key => $query ) {
			if ( 'relation' === $key ) {
				$cleaned_query['relation'] = $this->sanitize_relation( $query );

// First-order clause.
			} elseif ( self::is_first_order_clause( $query ) ) {

$cleaned_clause          = array_merge( $defaults, $query );
				$cleaned_clause['terms'] = (array) $cleaned_clause['terms'];
				$cleaned_query[]         = $cleaned_clause;

/*
				 * Keep a copy of the clause in the flate
				 * $queried_terms array, for use in WP_Query.
				 */
				if ( ! empty( $cleaned_clause['taxonomy'] ) && 'NOT IN' !== $cleaned_clause['operator'] ) {
					$taxonomy = $cleaned_clause['taxonomy'];
					if ( ! isset( $this->queried_terms[ $taxonomy ] ) ) {
						$this->queried_terms[ $taxonomy ] = array();
					}

/*
					 * Backward compatibility: Only store the first
					 * 'terms' and 'field' found for a given taxonomy.
					 */
					if ( ! empty( $cleaned_clause['terms'] ) && ! isset( $this->queried_terms[ $taxonomy ]['terms'] ) ) {
						$this->queried_terms[ $taxonomy ]['terms'] = $cleaned_clause['terms'];
					}

if ( ! empty( $cleaned_clause['field'] ) && ! isset( $this->queried_terms[ $taxonomy ]['field'] ) ) {
						$this->queried_terms[ $taxonomy ]['field'] = $cleaned_clause['field'];
					}
				}

// Otherwise, it's a nested query, so we recurse.
			} elseif ( is_array( $query ) ) {
				$cleaned_subquery = $this->sanitize_query( $query );

if ( ! empty( $cleaned_subquery ) ) {
					// All queries with children must have a relation.
					if ( ! isset( $cleaned_subquery['relation'] ) ) {
						$cleaned_subquery['relation'] = 'AND';
					}

$cleaned_query[] = $cleaned_subquery;
				}
			}
		}

return $cleaned_query;
	}

/**
	 * Sanitizes a 'relation' operator.
	 *
	 * @since 4.1.0
	 *
	 * @param string $relation Raw relation key from the query argument.
	 * @return string Sanitized relation. Either 'AND' or 'OR'.
	 */
	public function sanitize_relation( $relation ) {
		if ( 'OR' === strtoupper( $relation ) ) {
			return 'OR';
		} else {
			return 'AND';
		}
	}

/**
	 * Determines whether a clause is first-order.
	 *
	 * A "first-order" clause is one that contains any of the first-order
	 * clause keys ('terms', 'taxonomy', 'include_children', 'field',
	 * 'operator'). An empty clause also counts as a first-order clause,
	 * for backward compatibility. Any clause that doesn't meet this is
	 * determined, by process of elimination, to be a higher-order query.
	 *
	 * @since 4.1.0
	 *
	 * @param array $query Tax query arguments.
	 * @return bool Whether the query clause is a first-order clause.
	 */
	protected static function is_first_order_clause( $query ) {
		return is_array( $query ) && ( empty( $query ) || array_key_exists( 'terms', $query ) || array_key_exists( 'taxonomy', $query ) || array_key_exists( 'include_children', $query ) || array_key_exists( 'field', $query ) || array_key_exists( 'operator', $query ) );
	}

/**
	 * Generates SQL clauses to be appended to a main query.
	 *
	 * @since 3.1.0
	 *
	 * @param string $primary_table     Database table where the object being filtered is stored (eg wp_users).
	 * @param string $primary_id_column ID column for the filtered object in $primary_table.
	 * @return string[] {
	 *     Array containing JOIN and WHERE SQL clauses to append to the main query.
	 *
	 *     @type string $join  SQL fragment to append to the main JOIN clause.
	 *     @type string $where SQL fragment to append to the main WHERE clause.
	 * }
	 */
	public function get_sql( $primary_table, $primary_id_column ) {
		$this->primary_table     = $primary_table;
		$this->primary_id_column = $primary_id_column;

return $this->get_sql_clauses();
	}

/**
	 * Generates SQL clauses to be appended to a main query.
	 *
	 * Called by the public WP_Tax_Query::get_sql(), this method
	 * is abstracted out to maintain parity with the other Query classes.
	 *
	 * @since 4.1.0
	 *
	 * @return string[] {
	 *     Array containing JOIN and WHERE SQL clauses to append to the main query.
	 *
	 *     @type string $join  SQL fragment to append to the main JOIN clause.
	 *     @type string $where SQL fragment to append to the main WHERE clause.
	 * }
	 */
	protected function get_sql_clauses() {
		/*
		 * $queries are passed by reference to get_sql_for_query() for recursion.
		 * To keep $this->queries unaltered, pass a copy.
		 */
		$queries = $this->queries;
		$sql     = $this->get_sql_for_query( $queries );

if ( ! empty( $sql['where'] ) ) {
			$sql['where'] = ' AND ' . $sql['where'];
		}

return $sql;
	}

/**
	 * Generates SQL clauses for a single query array.
	 *
	 * If nested subqueries are found, this method recurses the tree to
	 * produce the properly nested SQL.
	 *
	 * @since 4.1.0
	 *
	 * @param array $query Query to parse (passed by reference).
	 * @param int   $depth Optional. Number of tree levels deep we currently are.
	 *                     Used to calculate indentation. Default 0.
	 * @return string[] {
	 *     Array containing JOIN and WHERE SQL clauses to append to a single query array.
	 *
	 *     @type string $join  SQL fragment to append to the main JOIN clause.
	 *     @type string $where SQL fragment to append to the main WHERE clause.
	 * }
	 */
	protected function get_sql_for_query( &$query, $depth = 0 ) {
		$sql_chunks = array(
			'join'  => array(),
			'where' => array(),
		);

$sql = array(
			'join'  => '',
			'where' => '',
		);

$indent = '';
		for ( $i = 0; $i < $depth; $i++ ) {
			$indent .= '  ';
		}

foreach ( $query as $key => &$clause ) {
			if ( 'relation' === $key ) {
				$relation = $query['relation'];
			} elseif ( is_array( $clause ) ) {

// This is a first-order clause.
				if ( $this->is_first_order_clause( $clause ) ) {
					$clause_sql = $this->get_sql_for_clause( $clause, $query );

$where_count = count( $clause_sql['where'] );
					if ( ! $where_count ) {
						$sql_chunks['where'][] = '';
					} elseif ( 1 === $where_count ) {
						$sql_chunks['where'][] = $clause_sql['where'][0];
					} else {
						$sql_chunks['where'][] = '( ' . implode( ' AND ', $clause_sql['where'] ) . ' )';
					}

$sql_chunks['join'] = array_merge( $sql_chunks['join'], $clause_sql['join'] );
					// This is a subquery, so we recurse.
				} else {
					$clause_sql = $this->get_sql_for_query( $clause, $depth + 1 );

$sql_chunks['where'][] = $clause_sql['where'];
					$sql_chunks['join'][]  = $clause_sql['join'];
				}
			}
		}

// Filter to remove empties.
		$sql_chunks['join']  = array_filter( $sql_chunks['join'] );
		$sql_chunks['where'] = array_filter( $sql_chunks['where'] );

if ( empty( $relation ) ) {
			$relation = 'AND';
		}

// Filter duplicate JOIN clauses and combine into a single string.
		if ( ! empty( $sql_chunks['join'] ) ) {
			$sql['join'] = implode( ' ', array_unique( $sql_chunks['join'] ) );
		}

// Generate a single WHERE clause with proper brackets and indentation.
		if ( ! empty( $sql_chunks['where'] ) ) {
			$sql['where'] = '( ' . "\n  " . $indent . implode( ' ' . "\n  " . $indent . $relation . ' ' . "\n  " . $indent, $sql_chunks['where'] ) . "\n" . $indent . ')';
		}

return $sql;
	}

/**
	 * Generates SQL JOIN and WHERE clauses for a "first-order" query clause.
	 *
	 * @since 4.1.0
	 *
	 * @global wpdb $wpdb The WordPress database abstraction object.
	 *
	 * @param array $clause       Query clause (passed by reference).
	 * @param array $parent_query Parent query array.
	 * @return array {
	 *     Array containing JOIN and WHERE SQL clauses to append to a first-order query.
	 *
	 *     @type string[] $join  Array of SQL fragments to append to the main JOIN clause.
	 *     @type string[] $where Array of SQL fragments to append to the main WHERE clause.
	 * }
	 */
	public function get_sql_for_clause( &$clause, $parent_query ) {
		global $wpdb;

$sql = array(
			'where' => array(),
			'join'  => array(),
		);

$join  = '';
		$where = '';

$this->clean_query( $clause );

if ( is_wp_error( $clause ) ) {
			return self::$no_results;
		}

$terms    = $clause['terms'];
		$operator = strtoupper( $clause['operator'] );

if ( 'IN' === $operator ) {

if ( empty( $terms ) ) {
				return self::$no_results;
			}

$terms = implode( ',', $terms );

/*
			 * Before creating another table join, see if this clause has a
			 * sibling with an existing join that can be shared.
			 */
			$alias = $this->find_compatible_table_alias( $clause, $parent_query );
			if ( false === $alias ) {
				$i     = count( $this->table_aliases );
				$alias = $i ? 'tt' . $i : $wpdb->term_relationships;

// Store the alias as part of a flat array to build future iterators.
				$this->table_aliases[] = $alias;

// Store the alias with this clause, so later siblings can use it.
				$clause['alias'] = $alias;

$join .= " LEFT JOIN $wpdb->term_relationships";
				$join .= $i ? " AS $alias" : '';
				$join .= " ON ($this->primary_table.$this->primary_id_column = $alias.object_id)";
			}

$where = "$alias.term_taxonomy_id $operator ($terms)";

} elseif ( 'NOT IN' === $operator ) {

if ( empty( $terms ) ) {
				return $sql;
			}

$terms = implode( ',', $terms );

$where = "$this->primary_table.$this->primary_id_column NOT IN (
				SELECT object_id
				FROM $wpdb->term_relationships
				WHERE term_taxonomy_id IN ($terms)
			)";

} elseif ( 'AND' === $operator ) {

if ( empty( $terms ) ) {
				return $sql;
			}

$num_terms = count( $terms );

$terms = implode( ',', $terms );

$where = "(
				SELECT COUNT(1)
				FROM $wpdb->term_relationships
				WHERE term_taxonomy_id IN ($terms)
				AND object_id = $this->primary_table.$this->primary_id_column
			) = $num_terms";

} elseif ( 'NOT EXISTS' === $operator || 'EXISTS' === $operator ) {

$where = $wpdb->prepare(
				"$operator (
					SELECT 1
					FROM $wpdb->term_relationships
					INNER JOIN $wpdb->term_taxonomy
					ON $wpdb->term_taxonomy.term_taxonomy_id = $wpdb->term_relationships.term_taxonomy_id
					WHERE $wpdb->term_taxonomy.taxonomy = %s
					AND $wpdb->term_relationships.object_id = $this->primary_table.$this->primary_id_column
				)",
				$clause['taxonomy']
			);

}

$sql['join'][]  = $join;
		$sql['where'][] = $where;
		return $sql;
	}

/**
	 * Identifies an existing table alias that is compatible with the current query clause.
	 *
	 * We avoid unnecessary table joins by allowing each clause to look for
	 * an existing table alias that is compatible with the query that it
	 * needs to perform.
	 *
	 * An existing alias is compatible if (a) it is a sibling of `$clause`
	 * (ie, it's under the scope of the same relation), and (b) the combination
	 * of operator and relation between the clauses allows for a shared table
	 * join. In the case of WP_Tax_Query, this only applies to 'IN'
	 * clauses that are connected by the relation 'OR'.
	 *
	 * @since 4.1.0
	 *
	 * @param array $clause       Query clause.
	 * @param array $parent_query Parent query of $clause.
	 * @return string|false Table alias if found, otherwise false.
	 */
	protected function find_compatible_table_alias( $clause, $parent_query ) {
		$alias = false;

// Confidence check. Only IN queries use the JOIN syntax.
		if ( ! isset( $clause['operator'] ) || 'IN' !== $clause['operator'] ) {
			return $alias;
		}

// Since we're only checking IN queries, we're only concerned with OR relations.
		if ( ! isset( $parent_query['relation'] ) || 'OR' !== $parent_query['relation'] ) {
			return $alias;
		}

$compatible_operators = array( 'IN' );

foreach ( $parent_query as $sibling ) {
			if ( ! is_array( $sibling ) || ! $this->is_first_order_clause( $sibling ) ) {
				continue;
			}

if ( empty( $sibling['alias'] ) || empty( $sibling['operator'] ) ) {
				continue;
			}

// The sibling must both have compatible operator to share its alias.
			if ( in_array( strtoupper( $sibling['operator'] ), $compatible_operators, true ) ) {
				$alias = preg_replace( '/\W/', '_', $sibling['alias'] );
				break;
			}
		}

return $alias;
	}

/**
	 * Validates a single query.
	 *
	 * @since 3.2.0
	 *
	 * @param array $query The single query. Passed by reference.
	 */
	private function clean_query( &$query ) {
		if ( empty( $query['taxonomy'] ) ) {
			if ( 'term_taxonomy_id' !== $query['field'] ) {
				$query = new WP_Error( 'invalid_taxonomy', __( 'Invalid taxonomy.' ) );
				return;
			}

// So long as there are shared terms, 'include_children' requires that a taxonomy is set.
			$query['include_children'] = false;
		} elseif ( ! taxonomy_exists( $query['taxonomy'] ) ) {
			$query = new WP_Error( 'invalid_taxonomy', __( 'Invalid taxonomy.' ) );
			return;
		}

if ( 'slug' === $query['field'] || 'name' === $query['field'] ) {
			$query['terms'] = array_unique( (array) $query['terms'] );
		} else {
			$query['terms'] = wp_parse_id_list( $query['terms'] );
		}

if ( is_taxonomy_hierarchical( $query['taxonomy'] ) && $query['include_children'] ) {
			$this->transform_query( $query, 'term_id' );

if ( is_wp_error( $query ) ) {
				return;
			}

$children = array();
			foreach ( $query['terms'] as $term ) {
				$children   = array_merge( $children, get_term_children( $term, $query['taxonomy'] ) );
				$children[] = $term;
			}
			$query['terms'] = $children;
		}

$this->transform_query( $query, 'term_taxonomy_id' );
	}

/**
	 * Transforms a single query, from one field to another.
	 *
	 * Operates on the `$query` object by reference. In the case of error,
	 * `$query` is converted to a WP_Error object.
	 *
	 * @since 3.2.0
	 *
	 * @param array  $query           The single query. Passed by reference.
	 * @param string $resulting_field The resulting field. Accepts 'slug', 'name', 'term_taxonomy_id',
	 *                                or 'term_id'. Default 'term_id'.
	 */
	public function transform_query( &$query, $resulting_field ) {
		if ( empty( $query['terms'] ) ) {
			return;
		}

if ( $query['field'] === $resulting_field ) {
			return;
		}

$resulting_field = sanitize_key( $resulting_field );

// Empty 'terms' always results in a null transformation.
		$terms = array_filter( $query['terms'] );
		if ( empty( $terms ) ) {
			$query['terms'] = array();
			$query['field'] = $resulting_field;
			return;
		}

$args = array(
			'get'                    => 'all',
			'number'                 => 0,
			'taxonomy'               => $query['taxonomy'],
			'update_term_meta_cache' => false,
			'orderby'                => 'none',
		);

// Term query parameter name depends on the 'field' being searched on.
		switch ( $query['field'] ) {
			case 'slug':
				$args['slug'] = $terms;
				break;
			case 'name':
				$args['name'] = $terms;
				break;
			case 'term_taxonomy_id':
				$args['term_taxonomy_id'] = $terms;
				break;
			default:
				$args['include'] = wp_parse_id_list( $terms );
				break;
		}

if ( ! is_taxonomy_hierarchical( $query['taxonomy'] ) ) {
			$args['number'] = count( $terms );
		}

$term_query = new WP_Term_Query();
		$term_list  = $term_query->query( $args );

if ( is_wp_error( $term_list ) ) {
			$query = $term_list;
			return;
		}

if ( 'AND' === $query['operator'] && count( $term_list ) < count( $query['terms'] ) ) {
			$query = new WP_Error( 'inexistent_terms', __( 'Inexistent terms.' ) );
			return;
		}

$query['terms'] = wp_list_pluck( $term_list, $resulting_field );
		$query['field'] = $resulting_field;
	}
}

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